Updates the header handling in the `Interceptor` and `RequestOverrider` with
the intention of mimicking the native behavior of `http.IncomingMessage.rawHeaders`.
> The raw request/response headers list exactly as they were received.

There are three fundamental changes in this changeset:

1) Header Input Type

Previously, headers could be provided to:
- `Scope.defaultReplyHeaders` as a plain object
- `Interceptor.reply(status, body, headers)` as a plain object or an array of raw headers
- `Interceptor.reply(() => [status, body, headers]` as a plain object

Now, all three allow consistent inputs where the headers can be provided as a
plain object, an array of raw headers, or a `Map`.

2) Duplicate Headers Folding

This change deviates from the suggested guidelines laid out in nock#1553 because
those guidelines didn't properly handle duplicate headers, especially when
some are defined as defaults.

This change was modeled to duplicate [Node's implementation](https://github.com/nodejs/node/blob/908292cf1f551c614a733d858528ffb13fb3a524/lib/_http_incoming.js#L245)
([relevant docs](https://nodejs.org/api/http.html#http_message_headers)).
It specifically lays out how duplicate headers are handled depending on the field name.

In the case of default headers, they are not included on the `Response`
(not even in the raw headers) if the field name exists in the reply headers
(using a case-insensitive comparison).

3) Raw Headers are the Source of Truth

Previously, the `Interceptor` and `RequestOverrider` mostly keep track of
headers as a plain object and the array of raw headers was created by looping
that object. This was the cause for inconsistencies with the final result of
the raw headers. The problem with that approach is that converting raw headers
to an object is a lossy process, so going backwards makes it impossible to
guarantee the correct results.

This change reverses that logic and now the `Interceptor` and `RequestOverrider`
maintain the header data in raw arrays. All additions to headers are only added
to raw headers. The plain headers object is never mutated directly, and instead
is [re]created from the raw headers as needed.

Co-Authored-By: Paul Melnikow <github@paulmelnikow.com>