Fix snake_cased oneof message are generated to incorrect types

[riku179]

snake_cased oneof message are not match generated js codes like below.
So I've fixed the messages are generated correctly.

sample.proto

syntax = "proto3";

message Sample {
    message msg_one {
    }

    message msg_two {
    }

    oneof msg_type {
        msg_one MSG_ONE = 1;
        msg_two MSG_TWO = 2;
    }
}

generated sample_pb.d.ts

...
export enum Msg_typeCase {
  MSG_TYPE_NOT_SET = 0,
  MSG_ONE = 1,
  MSG_TWO = 2,
}

generated sample_pb.js

/**
 * @enum {number}
 */
proto.Sample.MsgTypeCase = { 
  MSG_TYPE_NOT_SET: 0,
  MSG_ONE: 1,
  MSG_TWO: 2
};

[jonnyreeves]

Awesome, thanks for fixing this and adding tests; you can test out these changes by switching to 0.7.7-pre.34d3d68 on npm