test: test more http response splitting scenarios

The test verified the output of http.OutgoingMessage#writeHead() but
not http.OutgoingMessage#setHeader().  Also check the response body.

PR-URL: https://github.com/nodejs/node/pull/2945


Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Rod Vagg <r@va.gg>