do not raise AssertionError if the response is 0. 0 is False in Python 2.

Author: Yamashita Yuu

msgpackrpc/future.py

@@ -44,15 +44,15 @@ def result(self):
         return self._result
 
     def set_result(self, result):
-        assert result
+        assert result is not None
         self.set(result=result)
 
     @property
     def error(self):
         return self._error
 
     def set_error(self, error):
-        assert error
+        assert error is not None
         self.set(error=error)
 
     def attach_callback(self, callback):