Add the 'webpage_url' field to info_dict

The url for the video page, it must allow to reproduce the result.
It's automatically set by YoutubeDL if it's missing.

test/test_download.py

@@ -148,6 +148,9 @@ def generator(test_case):
                 # Check for the presence of mandatory fields
                 for key in ('id', 'url', 'title', 'ext'):
                     self.assertTrue(key in info_dict.keys() and info_dict[key])
+                # Check for mandatory fields that are automatically set by YoutubeDL
+                for key in ['webpage_url', 'extractor']:
+                    self.assertTrue(info_dict.get(key), u'Missing field: %s' % key)
         finally:
             try_rm_tcs_files()