Fixed type conversion issue with isProtocol method in RFC6455

I encountered the following error while attempting to use Ratchet. The __toString version of the request contained the correct value (13), so I cast the result to a string before casting it to an integer. 

Notice: Object of class Guzzle\Http\Message\Header could not be converted to int in [file system path]\vendor\cboden\ratchet\src\Ratchet\WebSocket\Version\RFC6455.php on line 53