Fixed #14711 -- Corrected the calculation for the previous month in generic...

Fixed #14711 -- Corrected the calculation for the previous month in generic date views. Thanks to msundstr for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15438 bcc190cf-cafb-0310-a4f2-bffc1f526a37