Fixed #12524 -- Clarified handling of pre-1000AD dates in datetime_safe (and...

Fixed #12524 -- Clarified handling of pre-1000AD dates in datetime_safe (and thus, the serializers). Patch includes moving the datetime_safe tests into the utils regressiontests module. Thanks to gsf for the report and initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12423 bcc190cf-cafb-0310-a4f2-bffc1f526a37