[1.2.X] Added protection against spoofing of X_FORWARDED_HOST headers. A security announcement will be made shortly.

      Backport of r16758 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@16764 bcc190cf-cafb-0310-a4f2-bffc1f526a37