public function headers($key = null, $default = null, $nullIfEmpty = false) {
        if ($this->headers === null) {
            $result = array(); 

            foreach ($this->serverVariables as $k => $v)
            {
                if (substr($k, 0, 5) == 'HTTP_') 
                {
                    $headerName = str_replace(' ', '-', ucwords(strtolower(str_replace('_', ' ', substr($k, 5)))));
                    $result[$headerName] = $v;
                }
            }

            $this->headers = $result;
        }

        return InternalHelper::getSingleKeyOrAll($this->headers, $key, $default, $nullIfEmpty);
    }