[release-branch.go1.15] net/http: update bundled x/net/http2
    
    Updates bundled http2 to x/net git rev 16c2bbf55 for:
    
    	http2: send a nil error if we cancel a delayed body write
    	https://golang.org/cl/288013
    
    	http2: wait until the request body has been written
    	https://golang.org/cl/288012
    
    Created by:
    
    go mod edit -replace=golang.org/x/net=golang.org/x/net@release-branch.go1.15-bundle
    GOFLAGS='-mod=mod' go generate -run=bundle std
    go mod edit -dropreplace=golang.org/x/net
    go get -d golang.org/x/net@release-branch.go1.15
    go mod tidy
    go mod vendor
    
    Fixes golang/go#42539
    
    Change-Id: I299c6d4a67ebc036e45c978e4d03cba73717b363
    Reviewed-on: https://go-review.googlesource.com/c/go/+/288112
    Trust: Damien Neil <dneil@google.com>
    Run-TryBot: Damien Neil <dneil@google.com>
    Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>