func (f FormResponse) Err() error {
	return &Error{
		RequestURI:   f.requestURI,
		ResponseCode: f.StatusCode,
		Code:         f.Get("error"),
		message:      f.Get("error_description"),
	}
}
Err returns an Error object extracted from the response.
package api
import (
	"encoding/json"
	"fmt"
	"io"
	"io/ioutil"
	"mime"
	"net/http"
	"net/url"
	"strconv"
)
type httpClient interface {
	PostForm(string, url.Values) (*http.Response, error)
}
type FormResponse struct {
	StatusCode int
	requestURI string
	values     url.Values
}
func (f FormResponse) Get(k string) string {
	return f.values.Get(k)
}