Fixed #14650 -- noted that underscores are also valid in slug fields. Thanks...

Fixed #14650 -- noted that underscores are also valid in slug fields.  Thanks to stringify for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14503 bcc190cf-cafb-0310-a4f2-bffc1f526a37