[1.2.X] Altered the behavior of URLField to avoid a potential DOS vector, and...

[1.2.X] Altered the behavior of URLField to avoid a potential DOS vector, and to avoid potential leakage of local filesystem data. A security announcement will be made shortly.

Backport of r16760 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@16766 bcc190cf-cafb-0310-a4f2-bffc1f526a37