Fix the method name clashes for generated commonjs files

[weilip1803]

This PR fixes #346.
When same the same proto file defines the same service name twice.

Example - test.proto

service Random
{
    rpc echo(google.protobuf.Empty) returns (stream SomeMessage) {}
}
service Random2
{
    rpc echo(google.protobuf.Empty) returns (stream SomeMessageTwo) {}
}

It will define the below method twice in the grpc_web_pb.js files which cause a error on run time

const methodInfo_echo = new grpc ....... 

[stanley-cheung]

Thanks for the fix!