If a server returns a DATA frame while procesing a HEAD request, the
client will discard the data.

Fixes golang/go#22376

Change-Id: Ief9c17ddfe51cc17f7f6326c87330ac9d8b9d3ff
Reviewed-on: https://go-review.googlesource.com/72551
Run-TryBot: Tom Bergan <tombergan@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tom Bergan <tombergan@google.com>