Fix #312, response xml visitor internals

This fixes #312 and allows for traversal of XML documents including nested attributes and namespaces.

 * [x] Remove implicit conversion of `SimpleXMLElement` to `array`
 * [x] Add traversal of XML document in `XMLVisitor`.
 * [ ] Investigate ways of working around `OperationResponseParser::visitAdditionalProperties()` limitations
 * [ ] Add XML namespace support traversal.