[release-branch.go1.7] net/http: update bundled http2

Updates bundled http2 for x/net/http2 git rev d4c55e66 for:

[release-branch.go1.7] http2: never Read from Request.Body in Transport
to determine ContentLength
https://golang.org/cl/31361

Updates #17480
Updates #17071

Change-Id: I2231adaed3cb5b368927a9654dcf7e69a8b664b6
Reviewed-on: https://go-review.googlesource.com/31432


Run-TryBot: Chris Broadfoot <cbro@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Chris Broadfoot <cbro@golang.org>