Various HttpServer and request handler updates

 * Added support for 'Expect: 100-continue'.
 * Reject HTTP > 1.1 requests with 505 HTTP Version Not Supported.
 * Changed the parse error field into an int and placed it inside a union,
   to conserve space and to make it possible to lookup the exact error.
 * Fix standards compliance of the Date header. OS X prints "UTC" instead
   of "GMT" as time zone, but this is not allowed by RFC 5322. Instead,
   we now use something like "+0200" as the time zone.