Fixed #7429 -- Modified the Session base class a little more dictionary-like...

Fixed #7429 -- Modified the Session base class a little more dictionary-like by adding update(), has_key(), values(), and iterator access methods. Thanks to Jeremy Dunck for the suggestion and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7687 bcc190cf-cafb-0310-a4f2-bffc1f526a37