static function resolveURLToData(string $url, ?array $requestHeaders = null) {
        if($requestHeaders === null) {
            $requestHeaders = array();
        }
        
        foreach($requestHeaders as $key => $val) {
            unset($requestHeaders[$key]);
            $nkey = \ucwords($key, '-');
            $requestHeaders[$nkey] = $val;
        }
        
        if(empty($requestHeaders['User-Agent'])) {
            $requestHeaders['User-Agent'] = static::DEFAULT_USER_AGENT;
        }
        
        $request = new \RingCentral\Psr7\Request('GET', $url, $requestHeaders);
        
        return static::makeRequest($request)->then(function ($response) {
            $body = (string) $response->getBody();
            return $body;
        });
    }