open xml files for defusedxml.minidom.parse (bug 997333)

https://bugzilla.mozilla.org/show_bug.cgi?id=997333

Since the filepath comes in as `unicode` and expatbuilder.py just [checks if it is an `str`](https://bitbucket.org/tiran/defusedxml/src/ac560aaf6f4a8c1e00b7961a11123d7580110056/defusedxml/expatbuilder.py?at=default#cl-87) to determine if it is a filepath this will manually open objects that don't look like file objects.