Cancel duplicate push promises streams

[dpwspoon]

If the http2-cache.js gets 2 push promises for the same url opened, it should cancel one of them.

It is possible, that when requests for the same resource are sent with different authorization headers, the response could be cache-control: public or some other shareable cache directive. This will result in multiple push promise streams for the same resource.

http2-cache.js should cancel one of the push promises if the request urls are the same

[hthetiot]

Need work on PR to handle cache directive a bit more i think