func (f DoerFunc) Apply(r *Requester) error {
	r.Doer = f
	return nil
}
Apply implements the Option interface.  DoerFuncs can be used as
requester options.  They install themselves as the requester's Doer.
package requester
import "net/http"
type Doer interface {
	Do(req *http.Request) (*http.Response, error)
}
type DoerFunc func(req *http.Request) (*http.Response, error)