public function addPostField($key, $value)
    {
        $this->setMethod('POST');
        $this->setBody(null);
        if (!is_array($value)) {
            $this->_postFields[$key] = strval($value);
        } else {
            $value = $this->formatArrayField($value);
            foreach ($value as $k => $v) {
                $k = $key . '[' . $k . ']';
                $this->_postFields[$k] = $v;
            }
        }
    }