json/context.go
func NewContext(timeout time.Duration) (Context, context.CancelFunc) {
	ctx, cancel := tchannel.NewContext(timeout)
	return tchannel.WrapWithHeaders(ctx, nil), cancel
}
func NewContext(timeout time.Duration) (Context, context.CancelFunc) {
	ctx, cancel := tchannel.NewContext(timeout)
	return tchannel.WrapWithHeaders(ctx, nil), cancel
}
// NewContext returns a Context that can be used to make JSON calls.