Fix 'no response handler with id' when using cider-test. 

Fixes https://github.com/clojure-emacs/cider/issues/853
No tests. How hard it would be to add integration-like test for `test.clj` middleware? I'm new to nrepl and cider so don't know. I'm talking about tests like "start nrepl server with test middleware, send messages to it and verify responses are correct". I can work on it.