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