Implement X-Cocaine-HTTP-Proto-Version logic

[noxiouz]

The X-Cocaine-HTTP-Proto-Version controls behavior of proxying replies from a cocaine application.
The idea was to allow to cocaine aplpications to force the proxy to finish a HTTP reply.

Usually proxy finishes HTTP reply as soon as Close message arrives (it's default behavior or if X-Cocaine-HTTP-Proto-Version == 1.0).
Alternative behavior is to finish HTTP reply on an empty body response (in case of X-Cocaine-HTTP-Proto-Version == 1.1)

https://github.com/cocaine/cocaine-tools/blob/master/cocaine/proxy/proxy.py#L758