Fixed #12544 and #13600 -- Fixed static files serving view to catch invalid...

Fixed #12544 and #13600 -- Fixed static files serving view to catch invalid date from If-Modified-Since header. Thanks akaihola and SmileyChris for patches.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13870 bcc190cf-cafb-0310-a4f2-bffc1f526a37