https-proxy-agent: augment the `opts` object for the `tls.connect` function

So that any additional options get picked up.
test: initial refactor of the tests to use "proxy" module
https-proxy-agent: use "extend" module

This allows us to default the "port" in a cleaner manner, as well as
we are now mixing in the `proxy` options to the destination server as well.

Mixing in the `proxy` options fixes the failing tests on node < node v0.11.6.
https-proxy-agent: more comments