http: throw if 'host' agent header is not a string value

If the 'host' agent header is an array or other non-string value, throw.

PR-URL: https://github.com/nodejs/node/pull/29568
Fixes: https://github.com/nodejs/node/issues/29408


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>