Fix wrong swagger schema generation when `response_body` is set.

[xclud]

When response_body is set in the proto file, the swagger still displays the schema from the response (actually MethodDescriptor.OutputType.ClrType) instead of response[response_body] while the data returned from the service is correct as expected.

This PR fixes the wrong swagger schema generation.

[dnfgituser]

All CLA requirements met.