Aeson tests don't validate toJSON and toEncoding agree

[dustin]

It's recommended to implement toEncoding when instantiating ToJSON, but there's no validation ensuring it agrees with toJSON (which is a requirement since it doesn't go the other way).

A simple workaround might be to encode toJSON v and v both while testing round tripping. They don't need to end with identical representations, but the decoder needs to agree that they're equivalent.

[NorfairKing]

Sounds good!

[NorfairKing]

PR welcome