Add a full url to negotiate response

[zackliu]

The full url like https://endpoint/path is suitable for engine.io to parse

Mainly used by engine.io negotiation's proposal: socketio/engine.io-client#710

[xingsy97]

What is the output of Engine.IO parsing? Is the output a pair of (endpoint, path)?
If given a single string url combining endpoint and path, how could the Engine.IO client distinguish which part is endpoint and which part is path?

For example, assuming url = "http://abc.com/foo/bar", there are 2 possible (endpoint, path) pairs

1. (endpoint, path) = ("http://abc.com", "/foo/bar")
2. (endpoint, path) = ("http://abc.com/foo", "/bar");
   Both of them are legal but only 1 is correct for server.

Let's sync about the negotiate design for native Socket.IO package later.