http: throw an error for unexpected agent values

As per https://github.com/nodejs/node/issues/9069
unexpected things can happen when supplying an
unexpected value to agent. Beings as the docs
clearly state the expected values, this throws
an error on an unexpected value.

PR-URL: https://github.com/nodejs/node/pull/10053


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>