Updates x/net/http2 to git rev 4d07e8a49 for CL 23287:

   http2: let handlers close Request.Body without killing streams
   https://golang.org/cl/23287

Fixes #15425

Change-Id: I20b6e37cd09aa1d5a040c122ca0daf14b8916559
Reviewed-on: https://go-review.googlesource.com/23301
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>