bearer/opts.go
func Transport(transport http.RoundTripper) Option {
	return func(tok *Bearer) error {
		tok.transport = transport
		return nil
	}
}
func Transport(transport http.RoundTripper) Option {
	return func(tok *Bearer) error {
		tok.transport = transport
		return nil
	}
}
// Transport is an option that sets an underlying client transport to the
// exchange process.