http2: don't send bogus :path pseudo headers if Request.URL.Opaque is set

Fixes golang/go#16847

Change-Id: I1e6ae1e0746051fd4cf7afc9beae7853293d5b68
Reviewed-on: https://go-review.googlesource.com/27632
Reviewed-by: Chris Broadfoot <cbro@golang.org>
3 files changed