Currently rpc.BlockNumber is marshalled to JSON as a numeric value, which is
wrong because BlockNumber.UnmarshalJSON() wants it to either be hex-encoded
or string "earliest"/"latest"/"pending". As a result, the call chain
rpc.BlockNumberOrHashWithNumber(123) -> json.Marshal() -> json.Unmarshal()
fails with error "cannot unmarshal object into Go value of type string".
