Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.7 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/tristan/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build824281085=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"

What did you do?

We are using google.JWTConfigFromJSON to read credentials from a google service account JSON config file. We're are then using these tokens with an internal service. This service requires the kid field in the JWT header, however x/oauth2/jwt does not set the header.

https://play.golang.org/p/xEDTBIsxXZ

If we use google.JWTAccessTokenSourceFromJSON, the correct headers are sent, but that provides no way to set scopes, or get an oauth2.Config

What did you expect to see?

kid header should be set.

What did you see instead?

kid header not set.