Fixed #17125 -- Made it possible to change a `RegexField`'s regular expression...

Fixed #17125 -- Made it possible to change a `RegexField`'s regular expression validator after it has been initialized. Thanks to Claude Paroz for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17076 bcc190cf-cafb-0310-a4f2-bffc1f526a37