try parse responseText when responseXML is null

Microsoft Edge returns null when Content-Type does not suit
see also: t-davies#5

Author: Hirohisa Yamaguchi

m-xml.js

@@ -63,7 +63,7 @@ THE SOFTWARE.
 
             xhr.open("GET", source, false);
             xhr.send();
-            return xhr.responseXML;
+            return xhr.responseXML || parseXMLString(xhr.responseText);
         }
     }