http2: add automatic gzip compression for the Transport

Tests are in net/http (clientserver_test.go, TestH12_AutoGzip)
from https://golang.org/cl/17241

Fixes golang/go#13298

Change-Id: I3f0b237ffdf6d547d57f29383e1a78c4f272fc44
Reviewed-on: https://go-review.googlesource.com/17242
Reviewed-by: Andrew Gerrand <adg@golang.org>
1 file changed