Revert change to the default Form.clean()

This means it doesn't break for people who are doing
`cleaned_data = super(FooForm, self).clean()`.

django/forms/forms.py

@@ -318,7 +318,7 @@ def clean(self):
         not be associated with a particular field; it will have a special-case
         association with the field named '__all__'.
         """
-        pass
+        return self.cleaned_data
 
     def has_changed(self):
         """

docs/releases/1.7.txt

@@ -129,8 +129,7 @@ Minor features
 
 * The :meth:`~django.forms.Form.clean` method on a form no longer needs to
   return ``self.cleaned_data``. If it does return a changed dictionary then
-  that will still be used. The default implementation no longer returns
-  ``self.cleaned_data``.
+  that will still be used.
 
 Backwards incompatible changes in 1.7
 =====================================

tests/forms_tests/tests/test_extra.py

@@ -620,6 +620,19 @@ def clean(self):
         self.assertTrue(f.is_valid())
         self.assertEqual(f.cleaned_data['username'], 'sirrobin')
 
+    def test_changing_cleaned_data_nothing_returned(self):
+        class UserForm(Form):
+            username = CharField(max_length=10)
+            password = CharField(widget=PasswordInput)
+
+            def clean(self):
+                self.cleaned_data['username'] = self.cleaned_data['username'].lower()
+                # don't return anything
+
+        f = UserForm({'username': 'SirRobin', 'password': 'blue'})
+        self.assertTrue(f.is_valid())
+        self.assertEqual(f.cleaned_data['username'], 'sirrobin')
+
     def test_changing_cleaned_data_in_clean(self):
         class UserForm(Form):
             username = CharField(max_length=10)