Backport sendRaw to 4.x

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Feature Request

I'm requesting sendRaw be backported to 4.x from 5.x for a number of reasons:

• There's an old codebase I have that relies on 4.x error structures and clients that would be incredibly laborious to migrate to 5.x
• I am attempting to integrate with a service that is still expecting XML responses (fairly large company that isn't going to update any time soon to json)

Use Case

There are a lot of old services that expect your API to respond with XML still, though their numbers are dwindling. While a lot of devs may have upgraded their restify to v5x+, my company has not.
It would be better to have this in the official module in my opinion, even if in an older version like 4x to prevent the requirement to fork the entire repo and potentially miss out on any update to 4x (albeit a very small chance of this since it's untouched for 5 years now) that could be around security.

Example API

Essentially the same as 5x sendRaw on responses

Are you willing and able to implement this?

Yes, there is an older PR that has the actual code change already for 5x: https://github.com/restify/node-restify/pull/1025/files

I have opened https://github.com/restify/node-restify/pull/1919/files for this request already. Thank you!

Start by reviewing the existing 5.x implementation referenced in pull request #1025 and the proposed 4.x changes in pull request #1919. Compare how sendRaw is exposed on responses in the 4.x codebase, then verify that the backport supports the XML-response use case described in this issue.