Types/coercible json naming change

[Krule]

Addressing: https://github.com/rom-rb/rom/pull/446#discussion_r133506000

Rename CoercibleMethods HashJson -> HashJSON, JsonHash -> JSONHash

Refactor attributes to keyword arguments.

• Add specs for usage of both individually.
• Add spec to cover case when Input JSON is invalid for JSONHash.

PR for 4.X will follow shortly.

@solnic I am also thinking about extending this a bit more by making it possible for JSON serializer to be plugged in. Any thoughts on idea? cc: @flash-gordon

[solnic]

Thanks man :)