[1.2.X] Fixed #14650 -- noted that underscores are also valid in slug fields. ...

[1.2.X] Fixed #14650 -- noted that underscores are also valid in slug fields.  Thanks to stringify for the report and patch.  Backport of [14503].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14504 bcc190cf-cafb-0310-a4f2-bffc1f526a37