Allow passing None to Form.set_renderer()

i.e. to remove any renderer which has been set

tailbone/forms2/core.py

@@ -576,6 +576,10 @@ class Form(object):
             self.enums.pop(key, None)
 
     def set_renderer(self, key, renderer):
+        if renderer is None:
+            if key in self.renderers:
+                del self.renderers[key]
+        else:
             self.renderers[key] = renderer
 
     def set_widget(self, key, widget):