http2: fix mapToHeaders() with single string value

This is for issue 16452. When 'set-cookie' header is set with an array
that has only one string value, it's split into its individual
characters.

Fix by resetting `isArray` to false when the value is converted from an
array to a string.

Fixes: https://github.com/nodejs/node/issues/16452
PR-URL: https://github.com/nodejs/node/pull/16458


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>