http: use `self.keepAlive` instead of `self.options.keepAlive`

In http.agent, all other options are directly accessed through
`self.` not `self.options`.

PR-URL: https://github.com/nodejs/node/pull/4407


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>