Converted sessions tests from doctest to unittest.

      Also made the FileSession backend consistent with other backends in one
corner case uncovered by the conversion, namely that the backend should
create a new key if the one passed in is invalid.



git-svn-id: http://code.djangoproject.com/svn/django/trunk@13482 bcc190cf-cafb-0310-a4f2-bffc1f526a37