[AspNetCoreModule] Use HttpResponse.PushPromise

[benaadams]

To more fully utilize http/2 where available, if response includes Link header with a local path and rel of preload add PushPromise for that path.

This will enable the reverse-proxy (IIS) to "value add" more http/2 features to Kestrel when used in that configuration. (e.g. Win10, Server 2016, nano server)

e.g.

Link: /css/site.css; rel=preload;

Add

HttpResponse.PushPromise("/css/site.css");

Or

Link:  /css/site.css; rel=preload; as=style;,/fonts/font1.woff; rel=preload; as=font;,/fonts/font2.woff; rel=preload; as=font;

Add

HttpResponse.PushPromise("/css/site.css");
HttpResponse.PushPromise("/fonts/font1.woff");
HttpResponse.PushPromise("/fonts/font2.woff");

IIS PushPromise as per http://blogs.iis.net/davidso/http2

Link header parsing to add push promises as per https://blog.cloudflare.com/announcing-support-for-http-2-server-push-2/

/cc @DamianEdwards

[d0pare]

Really must have feature.

Thanks @benaadams

[Tratcher]

RE: aspnet/HttpAbstractions#371

[Tratcher]

This issue was moved to aspnet/AspNetCoreModule#28