http: remove redundant code in _deferToConnect

Logic for calling the passed in socket method and/or callback
was duplicated. This commit refactors the relevant code to
remove the redundancy.

PR-URL: https://github.com/nodejs/node/pull/2769


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>