Add HeadersFrame.HasPriority accessor.
Add HeadersFrame.HasPriority accessor.

cmars/tools,cmars/tools,DramaFever/golang.org-x-net,DramaFever/golang.org-x-net,golang/net,gomini/net,raboof/net,albertjin/golang-net,cmars/tools,raboof/net,cesanta/goxnet,albertjin/golang-net,cesanta/goxnet,gomini/net,cmars/tools