interop: Fix unimplemented method test (#2040)

* Don't send nil requests.

* Fix import name and get rid of condition.

* Let registered encoder deal with nil requests.

* Break encode into encode and compress.

call_test.go

@@ -104,7 +104,7 @@ func (h *testStreamHandler) handleStream(t *testing.T, s *transport.Stream) {
 		}
 	}
 	// send a response back to end the stream.
-	data, err := encode(testCodec{}, &expectedResponse, nil, nil, nil)
+	data, err := encode(testCodec{}, &expectedResponse, nil)
 	if err != nil {
 		t.Errorf("Failed to encode the response: %v", err)
 		return