http: don't pass the request options to Agent

The `options` that were being passed in before here are specific to a
single request, which kinda defeats the purpose of using an Agent in the
first place.

On a worse note, these `options` have not yet been "processed" by the
`http.ClientRequest` class, so if `port: null` is set (like it is as the
result of a `url.parse()` call), then they take preference over the
processed values since the agent's "options" get mixed in last in the
`createSocket()` function.

Fixes #6197.
Fixes #6199.
Closes #6231.