XML cleanup, System.Xml.XPath.XDocument implementation and test coverage

- General XML cleanup (code reformatting, sanitizing, etc.),
- Replacing broken implementation of System.Xml.XPath.XDocument (previous one was find & replace after decoupling XPath.XDocument and XDocument and didn't work out)
- adding a little XDocument test coverage (more coming later)
- adding XPath tests (3 different implementations: XPathDocument, XDocument, XmlDocument)