http2: workaround TCPConn CloseWrite not being supported on Plan 9
http2: workaround TCPConn CloseWrite not being supported on Plan 9

This stops the tests from hanging on Plan 9.

Fixes golang/go#35904
Updates golang/go#17906

Change-Id: I2bcbb131629b217a99f9496cda0399ce21eb3020
Reviewed-on: https://go-review.googlesource.com/c/net/+/209417
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/net