Fix #312, #317, Response\*Visitor rewrite

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`.
 * [x] Investigate ways of working around `OperationResponseParser::visitAdditionalProperties()` limitations
 * [x] Add XML namespace support traversal.
 * [x] Add combination tests for OperationResponseParser
 * [x] Add complex nested XML structures' tests with attributes and various `additionalProperties` stances.
 * [x] Protect xml and json structures in Visitors
 * [x] Optimize XmlVisitor::xmlToArray()
 * [x] Incorporate support for top-level arrays and model `location` definition.
 * [x] Fix Response\HeaderVisitor with `additionalProperties`