Unclear how to send an empty response stream

[mzabaluev]

Steps to repro with 0.8.0 and master:

• Open the server implementation in the route_guide example in an editor.
• In the implementation of the list_features trait method, delete every line in the async closure passed to o.spawn(...) except the final expression calling resp.send_trailers(...).
• Build the example, run the server and the client.

This results in an internal error in the server sent as a response to the client:

thread 'main' panicked at 'feature: GrpcMessage(GrpcMessageError { grpc_status: 13, grpc_message: "grpc server handler did not close the sender" })', /somewhere/grpc-rust/grpc-examples/route_guide/src/client.rs:37:23

Same happens if the method does o.pump(stream::empty(), resp).