Fixed #736 -- Changed behavior of QueryDict items() to be more consistent, fixed mutability holes, gave MultiValueDict many more dictionary methods and added unit tests. Thanks, Kieran Holland. This is slightly backwards-incompatible if you happened to rely on the behavior of QueryDict.items(), which is highly unlikely.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@1504 bcc190cf-cafb-0310-a4f2-bffc1f526a37