HTTP: Parser: new parser->headers_extra array

The purpose of the new array, is to register unknown/custom headers
set by the client.

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
HTTP: Parser: add support for 'Upgrade' Header

Signed-off-by: Eduardo Silva <eduardo@monkey.io>
HTTP: Parser: parser now supports 'Upgrade' value on Connection header

Signed-off-by: Eduardo Silva <eduardo@monkey.io>