http: add type checking for hostname

Add type checking for options.hostname / options.host
Maintains the use of `undefined` and `null` for default `localhost`,
but other falsy values (like `false`, `0` and `NaN`) are rejected.

PR-URL: https://github.com/nodejs/node/pull/12494
Ref: https://github.com/nodejs/node/issues/12488


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>