Fixed #10581 -- Fixed conditional handling of If-Match headers.

The conditional processing decorator from r10114 wasn't parsing ETags
from an If-Match header correctly. Patch from Ivan Sagalaev (who also
did most of the work in r10114, before I rewrote parts of it and added
bonus bugs, although I forgot to thank him there).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10116 bcc190cf-cafb-0310-a4f2-bffc1f526a37