http: add callback is function check

We were checking that the callback existed, but not
checking that it was a function. In `setTimeout`, if
callback is truthy but not a function, throw a
TypeError

Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
PR-URL: https://github.com/nodejs/node/pull/3090