Deprecation warning in Node.js 12

[watson]

We need access to the host of an outgoing HTTP request and the easiest way to get this is from the Host header on the request object (req._headers.host).

Since the release of Node.js 12, the _headers property has been marked as deprecated (though it still works fine).

This results in Node.js printing:

(node:8970) [DEP0066] DeprecationWarning: OutgoingMessage.prototype._headers is deprecated

The deprecation happened in nodejs/node#24167 and as far as I can see, there's currently no workaround. I propose that we make a PR to Node.js to add a public way to retrieve the host.

[trentm]

This was resolved in #1417