Fixed #10458 -- Corrected the `next_month` and `previous_month` context variables provided with the generic month_archive view. The value returned now matches the docstring and the generic views documentation. Thanks to fperetti for the report and initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@10556 bcc190cf-cafb-0310-a4f2-bffc1f526a37