Fixed #14961 -- Revised staticfiles's prefix handling to make sure it runs on...

Fixed #14961 -- Revised staticfiles's prefix handling to make sure it runs on Windows. Also revised staticfiles tests to pass on Windows.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15119 bcc190cf-cafb-0310-a4f2-bffc1f526a37