Pass correct mode string to phpseclib

[suneth-perera]

• In the current process in cipher() method in JWE.php its $cipher = new AES(AES::MODE_CBC); where AES::MODE_CBC will be '2'.
• and according to the SymmetricKey.php constructor code shown below expects $mode to be a string. As the current code is passing 2, it endsup throw new BadModeException('No valid mode has been specified');

$mode = strtolower($mode);
// necessary because of 5.6 compatibility; we can't do isset(self::MODE_MAP[$mode]) in 5.6
$map = self::MODE_MAP;
if (!isset($map[$mode])) {
throw new BadModeException('No valid mode has been specified');
}