Index: django/http/__init__.py
===================================================================
--- django/http/__init__.py	(revision 8226)
+++ django/http/__init__.py	(working copy)
@@ -128,6 +128,10 @@
     Values retrieved from this class are converted from the given encoding
     (DEFAULT_CHARSET by default) to unicode.
     """
+
+    _mutable = True
+    encoding = None
+
     def __init__(self, query_string, mutable=False, encoding=None):
         MultiValueDict.__init__(self)
         if not encoding: