http: fix non-string header value concatenation

Since headers are stored in an empty literal object ({}) instead
of an object created with Object.create(null), care must be taken
with property names inherited from Object. Currently there are
only functions inherited, so we can safely check for existing
strings instead.

Fixes: https://github.com/nodejs/node/issues/4456
PR-URL: https://github.com/nodejs/node/pull/4460


Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>