http: optimize on_headers_complete

Use an array instead of an object to pass a parsed header chunk from c++
to javascript. This offers a 5-10% speedup on the http_simple benchmark,
as evidenced by running:

  ab -k -t 100 -c 100 http://127.0.0.1:8000/bytes/100

PR: https://github.com/iojs/io.js/pull/292


Reviewed-by: Ben Noordhuis <info@bnoordhuis.nl>