[1.2.X] Fixed #717 - If-Modified-Since handling should compare dates according to RFC 2616

Thanks to Maniac for the report, julienb for the initial patch, and
especially to aaugustin for the final patch and tests.

Backport of [15696] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15697 bcc190cf-cafb-0310-a4f2-bffc1f526a37