EncoderError: derive PartialEq

[Keruspe]

Makes writing serialization tests easier

[alexcrichton]

Thanks!

[BurntSushi]

@alexcrichton FWIW, I believe this PR bumped the minimum Rust version of this crate to 1.10. Namely, rustc-serialize 0.3.19 compiles on Rust 1.9 but rustc-serialize 0.3.20 does not. This particular PR adds derive(PartialEq) which in turn requires PartialEq on std::fmt::Error, but that was added in 3df35a in rust-lang/rust, which I think first became available in Rust 1.10.

I think there is still an open question whether this qualifies as a breaking change (I still think it does), but it breaks builds and forces me to either forcefully pin down rustc-serialize 0.3.19 or bump the minimum Rust version required. (I don't think I am tied to 1.9 at the moment, so maybe I should just upgrade, but I feel like it's important to acknowledge that this is a frustration.)

[BurntSushi]

(To be clear, my build broke because I ran cargo update locally and didn't test the change locally on Rust 1.9.)