http: Improve HTTP Expect header handling

[faridnsh]

Just following the spec:
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.20

Fixes #4651

Note that, Now we have the following pattern 3 times in the _http_server.js:

if (EventEmitter.listenerCount(self, 'someEventName') > 0) {
    self.emit('someEventName');
} else {
     // Do something as default
}

I think maybe we should move it to a function, though didn't have any good idea for the name of the function. Something like this. If you guys give me a good name, I'll refactor it.

I also see this pattern in userland too. Maybe it should be a event emitter prototype method name setDefaultListener that will set the default action when there's no listeners but won't do anything when there are listeners to a particular event.

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[tjfontaine]

I'm generally ok with this, but I want to solicit some feedback from others using this feature to know if this would be too much of a surprise.

On the pattern, you could describe it this way:

if (!self.emit('checkExpectation'...)) {
  res.writeHead(417);
  res.end();
}

I'm not really thrilled with the idea of the prototype method.

[jasnell]

@alFReD-NSH ... still want this?

[faridnsh]

I actually don't need this for myself anymore. But should check with #4651

[jasnell]

Ok, given the change I'm not sure if this would land in here or if it should be retargeted to nodejs/node. @alFReD-NSH , are you willing to take another look at this?

[faridnsh]

I don't need this anymore. If you think this should be merged, then merge it if not, if it needs any changes, then close this, because at this moment I don't have time for this.

[jasnell]

For now I'll mark it as a low priority and assign myself to it. If I'm able to get back to it I will, otherwise it'll just have to sit a while longer. Thanks @alFReD-NSH !

[jasnell]

Opened a new issue in nodejs/node to track this. Will close this PR as it's not going to be able to land.