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.
contextify: fix ContextifyContext leak

Apparently, context->Global() won't be destroyed if the context itself
isn't marked as weak and independent.

Also, the weakness flag should be cleared once the weak callback is
executed, otherwise we'll get crashes in Debug builds.

fix #6115 and #6201