Fixed #9522 -- Modified handling of values in base serializer so that field subclasses can define their own value_to_string() method for serialization. Thanks to Alex Koshelev for the report and initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10554 bcc190cf-cafb-0310-a4f2-bffc1f526a37