REP-1283 DeRP Filter

I'm not sure if system level tests will be necessary for this story? Since I can actually test the Java filter API doFilter method, my unit tests are fairly comprehensive and can be expanded to cover more cases. Actually, I suppose we need a test for the response messaging service, at least.

Note: The tests won't pass on this branch until we cut a release of http-delegation. We should also probably update the version api-checker uses.
I just want to get eyes on this since I'm not sure this is the "right" way to send a response with a body. Do we want to set the content type to `text/plain`?