http2: simplify mapToHeaders, stricter validation

No longer check whether key is a symbol as Object.keys does not
return symbols. No longer convert key to string as it is always
a string. Validate that only one value is passed for each
pseudo-header.

Extend illegal connection header message to include the name of
the problematic header.

Extend tests to cover this behaviour.

PR-URL: https://github.com/nodejs/node/pull/16575


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>