Fix window resizing of solidus_chrome_headless driver

Fixes errors similar to the one below when running
`solidus_chrome_headless` in a Solidus extension:

```
  1) Checkout leaving and returning to address step
     Got 0 failures and 2 other errors:

     1.1) Failure/Error: visit spree.root_path

          NoMethodError:
            undefined method `args' for nil:NilClass
          # ./spec/features/checkout_spec.rb:25:in `block (2 levels) in <top (required)>'
          # ./spec/support/confirm_helpers.rb:14:in `block (2 levels) in <top (required)>'
```

According to #183:

> After updating to cimg/ruby:2.7-browsers, we have a newer version of
Selenium running on CircleCI. As such, we need to define our
configuration in a way that is compatible with the new version.

Author: gsmendoza

lib/solidus_dev_support/rspec/capybara.rb

@@ -4,6 +4,8 @@
 
 # Allow to override the initial windows size
 CAPYBARA_WINDOW_SIZE = ENV.fetch('CAPYBARA_WINDOW_SIZE', '1920x1080').split('x', 2).map(&:to_i)
+CAPYBARA_WINDOW_WIDTH = CAPYBARA_WINDOW_SIZE[0]
+CAPYBARA_WINDOW_HEIGHT = CAPYBARA_WINDOW_SIZE[1]
 
 Capybara.javascript_driver = ENV.fetch('CAPYBARA_JAVASCRIPT_DRIVER', "solidus_chrome_headless").to_sym
 Capybara.default_max_wait_time = 10
@@ -12,7 +14,9 @@
 Capybara.drivers[:selenium_chrome_headless].tap do |original_driver|
   Capybara.register_driver :solidus_chrome_headless do |app|
     original_driver.call(app).tap do |driver|
-      driver.options[:options].args << "--window-size=#{CAPYBARA_WINDOW_SIZE.join(',')}"
+      driver.resize_window_to(
+        driver.current_window_handle, CAPYBARA_WINDOW_WIDTH, CAPYBARA_WINDOW_HEIGHT
+      )
     end
   end
 end