Fixed #14695 -- Stopped model fields from incorrectly overwriting the field...

Fixed #14695 -- Stopped model fields from incorrectly overwriting the field name during model initialization if it was already passed as a keyword argument. Thanks, erikrose and willhardy.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16614 bcc190cf-cafb-0310-a4f2-bffc1f526a37