Fixes #13804 -- URLField validation failure for a url containing '://' on the...

Fixes #13804 -- URLField validation failure for a url containing '://' on the path and no scheme. Backport of r14657

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14658 bcc190cf-cafb-0310-a4f2-bffc1f526a37