http-proxy-test: local var instead of '.bind(this)'; add comment
improve tests: local var instead of '.bind(this)'; add proper guards for unexpected code path in tests involving promises
improve tests: fix glitches re "null" vs "undefined"; consistent var names: a resource is always "res", the result of ".serialize(..)" is always "serialized"; "rejects" instead of "fails" where appropriate; simplified a few tests by using assert.content
Merge branch '#336_improve_tests' into DONT_MERGE_ME_#336_temp