[1.0.X] Fixed #9199 -- We were erroneously only prepending "www" to the domain

if we also needed to append a slash (when PREPEND_WWW=True).

Based on a patch and tests from gonz. Thanks.

Backport of r9184 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.0.X@9185 bcc190cf-cafb-0310-a4f2-bffc1f526a37