test: add tests for add/remove header after sent

This change adds tests to make sure an Error is thrown if a header
is added or removed after they are sent.

PR-URL: https://github.com/nodejs/node/pull/8682


Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>