Fix orphan-removal in XML driver

Issue: When using XML mapping, the "orphan-removal" attribute is stored in the ClassMetadata as a SimpleXML object instead of a boolean. This leads Doctrine to consider orphan-removal as enabled as soon as you provide an "orphan-removal" attribute, even if you specify orphan-removal="false".
 
Solution: The XML driver was mapping the wrong value. I've added a test case since it was missing for orphan-removal, and i've also updated the XSD file with the orphan-removal attribute.