Fix World Grain date parser

[mkao006]

There appears to be multiple erros when parsing the world grain dates.

This may be considered a minor bug, and the effort may not be justified.

2018-06-22 15:32:25 [scrapy.core.scraper] ERROR: Spider error processing <GET http://www.world-grain.com/articles/news_home/Site_News/2012/08/GRFA_responds_to_IFPRI_report.aspx?ID=%7B4791C417-B261-47A3-9778-DFD017DF1905%7D&p=1> (referer: http://www.world-grain.com/articles/news_home/Site_News/2012/08/GRFA_responds_to_IFPRI_report.aspx?ID=%7B4791C417-B261-47A3-9778-DFD017DF1905%7D)
Traceback (most recent call last):
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/utils/defer.py", line 102, in iter_errback
    yield next(it)
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/offsite.py", line 30, in process_spider_output
    for x in result:
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/referer.py", line 339, in <genexpr>
    return (_set_referer(r) for r in result or ())
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/urllength.py", line 37, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/depth.py", line 58, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spiders/crawl.py", line 76, in _parse_response
    cb_res = callback(response, **cb_kwargs) or ()
  File "/home/mk/Git/fao/est/TheReadingMachine/thereadingmachine/scraper/news_scraper/spiders/spiders.py", line 216, in parse_item
    clean_date = self._parse_wg_date(self._clean_date(raw_date))
  File "/home/mk/Git/fao/est/TheReadingMachine/thereadingmachine/scraper/news_scraper/spiders/spiders.py", line 160, in _parse_wg_date
    raise ValueError('no valid date format found for ' + ds)

or date with different format,

2018-06-22 15:34:42 [scrapy.core.scraper] ERROR: Spider error processing <GET http://www.world-grain.com/articles/news_home/World_Grain_News/2013/07/All_US_wheat_production_drops.aspx?ID=%7B3BBC54BE-3C53-4136-8C10-8B621C7E05EC%7D> (referer: http://www.world-grain.com/News/Archive.aspx?page=19&year=2013&month=0)
Traceback (most recent call last):
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/utils/defer.py", line 102, in iter_errback
    yield next(it)
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/offsite.py", line 30, in process_spider_output
    for x in result:
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/referer.py", line 339, in <genexpr>
    return (_set_referer(r) for r in result or ())
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/urllength.py", line 37, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/depth.py", line 58, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spiders/crawl.py", line 76, in _parse_response
    cb_res = callback(response, **cb_kwargs) or ()
  File "/home/mk/Git/fao/est/TheReadingMachine/thereadingmachine/scraper/news_scraper/spiders/spiders.py", line 216, in parse_item
    clean_date = self._parse_wg_date(self._clean_date(raw_date))
  File "/home/mk/Git/fao/est/TheReadingMachine/thereadingmachine/scraper/news_scraper/spiders/spiders.py", line 160, in _parse_wg_date
    raise ValueError('no valid date format found for ' + ds)
ValueError: no valid date format found for Jul 12, 2013

In addition, sometimes the extracted date is empty which results in the following error.

2018-06-22 15:32:31 [scrapy.core.scraper] ERROR: Spider error processing <GET http://www.world-grain.com/errors/404.aspx?item=%2farticles%2fnews_home%2fsite_news%2f2012%2f08%2fgrfa_responds_to_ifpri_report&user=extranet%5cAnonymous&site=wg> (referer: http://www.world-grain.com/articles/news_home/Site_News/2012/08/GRFA_responds_to_IFPRI_report.aspx?ID=%7B4791C417-B261-47A3-9778-DFD017DF1905%7D)
Traceback (most recent call last):
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/utils/defer.py", line 102, in iter_errback
    yield next(it)
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/offsite.py", line 30, in process_spider_output
    for x in result:
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/referer.py", line 339, in <genexpr>
    return (_set_referer(r) for r in result or ())
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/urllength.py", line 37, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spidermiddlewares/depth.py", line 58, in <genexpr>
    return (r for r in result or () if _filter(r))
  File "/home/mk/Git/fao/est/TheReadingMachine/venv/local/lib/python2.7/site-packages/scrapy/spiders/crawl.py", line 76, in _parse_response
    cb_res = callback(response, **cb_kwargs) or ()
  File "/home/mk/Git/fao/est/TheReadingMachine/thereadingmachine/scraper/news_scraper/spiders/spiders.py", line 216, in parse_item
    clean_date = self._parse_wg_date(self._clean_date(raw_date))
  File "/home/mk/Git/fao/est/TheReadingMachine/thereadingmachine/scraper/news_scraper/spiders/spiders.py", line 163, in _clean_date
    clean = (raw.replace('\t', '')
AttributeError: 'NoneType' object has no attribute 'replace'