Allow enum variant as hashmap key

[oli-obk]

merge after #31
closes rust-lang/rust#21470 here

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[huonw]

Does this work with Decodable?

[oli-obk]

huonw: that's how it worked before, but i'll add a test

[oli-obk]

@huonw i changed the test to also check if deserialize still works.
actually i think rust will decode almost everything, even invalid json like {0:"hi"} without complaint. It just won't create invalid json

[alexcrichton]

Thanks!