http: remove unnecessary check

The value passed to `process.nextTick()` which is passed to the
callback is already a valid object, so the conditional will always
evaluate to true.

PR-URL: https://github.com/nodejs/node/pull/5233


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>