Support HTTP Upgrade (low priority)

[danja]

See https://en.wikipedia.org/wiki/HTTP/1.1_Upgrade_header

It's quite an obscure and probably rarely implemented header, essentially for changing protocol (request initiated by client). But I think it could potentially be very useful e.g. for shifting to a streaming protocol for media or some kind of fast-but-unreliable data transfer via UDP - triple per packet..?

I'm quite interesting in playing with it from a RESTful agents kind of perspective, but please consider this low priority.

(I haven't checked if OPTIONS and PATCH are supported yet - they'd both be much higher priority)