Fix file Xml to XML

This fixes operating system that have file case sensitivity and out the following:

```
Fatal error: Class 'Silverpop\Util\ArrayToXML' not found in /home/ckdarby/www/silverpop/vendor/simpleweb/silverpopphp/src/Silverpop/EngagePod.php on line 641

Call Stack:
    0.0002     227464   1. {main}() /home/ckdarby/www/silverpop/test.php:0
    0.0028     402728   2. Silverpop\EngagePod->__construct() /home/ckdarby/www/silverpop/test.php:19
    0.0028     402880   3. Silverpop\EngagePod->_login() /home/ckdarby/www/silverpop/vendor/simpleweb/silverpopphp/src/Silverpop/EngagePod.php:34
    0.0028     404344   4. Silverpop\EngagePod->_request() /home/ckdarby/www/silverpop/vendor/simpleweb/silverpopphp/src/Silverpop/EngagePod.php:613
```

If you composer install this on Ubuntu you'll be prompted with the above giant error.