http: docs-only deprecation of res.writeHeader()

Doc-only deprecation of the undocumented res.writeHeader() API.
Also makes res.writeHeader an alias of res.writeHead since the
previous implementation simply deferred to that method.

PR-URL: https://github.com/nodejs/node/pull/11355


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>