Pull in approved backports to golang.org/x/net/http2:

	d8c3cde set ContentLength to -1 for HEAD response with no Content-Length
	7b24c0a set Response.ContentLength to 0 when headers end stream
	c4031f5 don't abort half-closed streams on server connection close
	2f744fa on write errors, close ClientConn before returning from RoundTrip
	275be3f deflake TestTransportReqBodyAfterResponse_200
	d26011a close the Request's Body when aborting a stream
	e5dd05d return unexpected eof on empty response with non-zero content length
	640e170 don't rely on system TCP buffer sizes in TestServer_MaxQueuedControlFrames
	198b78c detect write-blocked PING frames
	20ed279 avoid race in TestTransportReqBodyAfterResponse_403.
	d585ef0 avoid clientConnPool panic when NewClientConn fails
	d06dfc7 avoid extra GetConn trace call
	1760f31 refactor request write flow
	6e87631 remove PingTimeout from TestTransportPingWhenReading
	b843c7d fix Transport connection pool TOCTOU max concurrent stream bug
	ab1d67c shut down idle Transport connections after protocol errors
	3741e47 remove check for read-after-close of request bodies
	2df4c53 fix race in DATA frame padding refund
	d7eefc9 avoid blocking while holding ClientConn.mu
	78e8d65 fix off-by-one error in client check for max concurrent streams
	828651b close request body after early RoundTrip failures
	59c0c25 limit client initial MAX_CONCURRENT_STREAMS
	524fcad make Transport not reuse conns after a stream protocol error
	0fe5f8a accept zero-length block fragments in HEADERS frames
	0e5043f close the request body if needed
	bb4ce86 reduce frameScratchBuffer caching aggressiveness
	3112343 also set "http/1.1" ALPN in ConfigureServer
	63939f4 switch to ASCII equivalents of string functions
	54161af use (*tls.Dialer).DialContext in dialTLS
	75b906f discard DATA frames with higher stream IDs during graceful shutdown
	1dfe517 rework Ping test to rely less on timing

By doing:

	$ go get -d golang.org/x/net@internal-branch.go1.16-vendor
	go get: upgraded golang.org/x/net v0.0.0-20210901185431-d2e9a4ea682f => v0.0.0-20211101194150-d8c3cde3c676
	$ go mod tidy
	$ go mod vendor
	$ go generate -run=bundle std

Fixes #49076.
Fixes #48822.
Fixes #48649.

Change-Id: Ie17f327eef2b6e6a9a1ac7635c5c4daef792e893
Reviewed-on: https://go-review.googlesource.com/c/go/+/359774
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>