code: public OkRequest<T> form(final Map.Entry<String, String> entry) {
          return form(entry, CHARSET_UTF8);
      }
docs: Write the key and value in the entry as form data to the request body
  <p/>
  The pair specified will be URL-encoded in UTF-8 and sent with the
  'application/x-www-form-urlencoded' content-type
  
  @param entry
  @return this request