Fixed #12386 -- Corrected the column names produced for autogenerated m2m...

Fixed #12386 -- Corrected the column names produced for autogenerated m2m tables when the related table is specified as a dot-separated string. Thanks to ldevesine for the report and simonb for the extra test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12226 bcc190cf-cafb-0310-a4f2-bffc1f526a37