Comment 4 for bug 1256218

Thanks for the patches and test. I've merged in the latest one, in combination with my patch from earlier. It's fair to say I'd have done it in a more general and complicated way, but this patch fixes the problem in hand and doesn't really have any drawbacks, so I've committed it - we can worry about the general case when we need to. The mutable_mapping test now works, hopefully you can check if the requests test does too.