Come up with proper solution for keys being strings or atoms

[seanabrahams]

In practice, clientMutationID is null on mutation responses due to the code expecting an atom key but receiving a string key.

I've updated the codebase to check for both but this "dirtiness" is not acceptable.

https://github.com/graphql-elixir/graphql-relay-elixir/blob/master/lib/graphql/relay/mutation.ex#L49