Expect/100-continue support

[mnot]

This branch adds server- and client-side support for HTTP/1.1 Expect/continue.

This is important because some clients (e.g., Curl) will send Expect/Continue on requests with bodies, and by default will pause until either a) they see a 100 Continue response, or b) they time out (1-3 secs, IME). Since node doesn't handle Expect/continue, it introduces an unnecessary delay.

On the server side, it creates a new event, 'check_continue' that is emitted when a request contains 'Expect: 100-continue'.

The event is passed request and response objects, just like the 'request' event. If the server wants the request to continue, it calls response.sendContinue(); if not, it sends a final status code. Note that 'request' is not fired if 'check_continue' is; see the test case for an example.

If the event is not handled, node will automagically generate the 100 Continue when appropriate. This is IMO the most reasonable default behaviour, while still allowing full control over the feature.

On the client side, it creates a new 'continue' event that's fired if a 100 Continue is received.

This probably needs a bit more testing around the corner cases to make sure it always does the right thing; I'll try to do so and report back. Just figured it would be good to get some review and others' experience in the meantime.

[ry]

Thanks! Finally proper 100-continue support!

Committed in d59512f.

The only modification was squashing and:

:%s/sendContinue/writeContinue/g
:%s/check_continue/checkContinue/g