Fixed #11670 -- Prevented genuine model fields named 'year', 'month', 'gt',...

Fixed #11670 -- Prevented genuine model fields named 'year', 'month', 'gt', 'lt' etc. from being mistaken for lookup types in lookups across relations. Thanks to andy for the report, to jpwatts for the initial patch and to Anssi Kääriäinen and Alex Gaynor for the reviews.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17450 bcc190cf-cafb-0310-a4f2-bffc1f526a37