Fix POST mutability problem

Originally, the code would make GET mutable and then change the value in the GET QueryDict. I changed it to change the value in the POST QueryDict with the other data because otherwise it wasn't setting happy right.

However, I screwed up and didn't make the POST QueryDict mutable. I'm not entirely sure how that passed all the testing. My best guess is that request.GET/request.POST are different in the tests than they are than on a server. And for some reason, you can add a `foo = 0` to an imutable POST QueryDict, but can't add a `foo = 1`. Since I was only testing the sad case (that's the only one you can test with about:feedback), that's the only case I was triggering.

Totally puzzling.

Tiny r?