Fix #200: pass schema location to MSXMLWrapper::checkValidity

[haja-fgabriel]

Related to issue #200. Notepad++ complained on some XML documents I have been working with at work that it has not found a valid DTD or schema, despite having the "xmlns" namespace declaration 😒 I will add support for namespaced schema locations, because I couldn't figure it out in 5 minutes, since I didn't have the repository cloned on the work laptop when I opened the PR.

@morbac what do you think?