Usage of OutgoingMessage._headers is deprecated

[sergburn]

https://github.com/restify/node-restify/blob/9153587c023a876237c1d8bc7491fee4984d9074/lib/response.js#L410

When 'trace'-level logging is enabled, access to self._headers causes a deprecation warning like this:

internal/process/warning.js:132
      throw warning;
      ^

DeprecationWarning: OutgoingMessage.prototype._headers is deprecated
    at ServerResponse.__send (/server/node_modules/restify/lib/response.js:410:31)
    at ServerResponse.send (/server/node_modules/restify/lib/response.js:316:21)
    at emitError (/server/node_modules/restify/lib/server.js:1447:17)
    at onResult (/server/node_modules/restify/lib/server.js:1528:20)
    at next (/server/node_modules/vasync/lib/vasync.js:843:6)
    at emitErrDone (/server/node_modules/restify/lib/server.js:1521:28)
    ...
  name: 'DeprecationWarning',
  code: 'DEP0066'
}

Node.js version 12.16.1.

It is especially a headache if throw-on-deprecation is enabled, for example as mocha ... --throw-deprecation

Could this be fixed still for 8.x?