[1.2.X] Fixed #14812 -- Made parsing of the If-Modified-Since HTTP header more...

[1.2.X] Fixed #14812 -- Made parsing of the If-Modified-Since HTTP header more robust in presence of malformed values when serving static content. Thanks shaohua for the report, and alexey.smolsky@gmail.com for a similar report and patch.
Backport of r14753 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14754 bcc190cf-cafb-0310-a4f2-bffc1f526a37