http2: set ContentLength to -1 for HEAD response with no Content-Length
http2: set ContentLength to -1 for HEAD response with no Content-Length

When reading a response to a HEAD request with no Content-Length,
set the Response ContentLength to -1 (unknown) rather than 0.

Fixes failing net/http TestH12_HeadContentLengthNoBody.

Change-Id: I472cbcf555e7f70bc41f58bf96da3d3c5bed9654
Reviewed-on: https://go-review.googlesource.com/c/net/+/360376
Trust: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
Run-TryBot: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/net