getStreamingResponseHeader errors when header not in Access-Control-Expose-Headers

[pnegahdar]

https://github.com/grpc/grpc-web/blob/master/javascript/net/grpc/web/grpcwebclientreadablestream.js#L122

The invocations fail when the grpc-status is exposed in the payload (not header) per the spec. Since the grpc-status and grpc-message headers are no longer declared in the Access-Control-Expose-Headers header, accessing it from the Xhrio lib will give an error instead of returning null

These are related to the issues pointed out in #518 & improbable-eng/grpc-web#396

Suggested fix: first see if grpc-status and grpc-message are in the getResponseHeaders() results before calling getStreamingResponseHeader.

I'd fix this if I had any idea how to build this project.

[stanley-cheung]

@pnegahdar Sorry for being late to look into this.

I think what you said make sense, but how do I get into a situation where "the grpc-status is exposed in the payload (not header) per the spec"?