Authorization decorator

Our first decorator is Authorization. We call the Header helper function that adds the Authorization header with the given token to each request at runtime:

func Authorization(token string) Decorator {
       return Header("Authorization", token)
}

func Header(name, value string) Decorator {
       return func(c Client) Client {
              return ClientFunc(func(r *http.Request)(*http.Response, error) {
                     r.Header.Add(name, value)
                     return c.Do(r)
              })
       }
}