[1.1.X] Fixed #11012: don't needless convert cache values to unicode.

This means you can now successfully store binary blogs, such as compressed data,
in the cache.

Thanks to Matt Croydon for the final patch.

Backport of [12637] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12638 bcc190cf-cafb-0310-a4f2-bffc1f526a37