The DuplicateVoteEvidence struct is missing JSON tags, resulting in camel case JSON output.

Discovered while adding support for this in the tendermint-rs: https://github.com/informalsystems/tendermint-rs/pull/242/files#r417475910

I wonder if there's other types that might be missing JSON tags and if there's some easy way to check for that?

Would be breaking on the RPC to fix this, but also doubles as a bug fix since JSON should only use snake_case