BUG: Triage and Fix interoperability issue with grpc-web mode:grpcwebtext

[Rkiouak]

Currently js client code generated via the protoc --js_out --grpc-web_out=...,mode=grpcwebtext fails to invoke callbacks, or recognize data.

js client code generated with protoc --js_out --grpc-web_out=...,mode=grpcweb appears to work as expected.

Identify and resolve root cause of this issue with grpc-web's mode=grpcwebtext