Use flat object instead of array-of-arrays for HTTP headers.

Use flat object instead of array-of-arrays for HTTP headers.

E.G. { "Content-Length": 10, "Content-Type": "text/html" } instead of
[["Content-Length", 10], ["Content-Type", "text/html"]].
The main reason for this change is object-creation efficiency.

This still needs testing and some further changes (like when receiving
multiple header lines with the same field-name, they are concatenated with a
comma but some headers ("Content-Length") should not be concatenated ; the
new header line should replace the old value).

Various thoughts on this subject:
http://groups.google.com/group/nodejs/browse_thread/thread/9a67bb32706d9efc#
http://four.livejournal.com/979640.html
http://mail.gnome.org/archives/libsoup-list/2009-March/msg00015.html