add model for xml-js

javascript/ql/test/library-tests/TaintTracking/BasicTaintTracking.expected

@@ -148,3 +148,4 @@ typeInferenceMismatch
 | xml.js:5:18:5:25 | source() | xml.js:8:14:8:17 | text |
 | xml.js:12:17:12:24 | source() | xml.js:13:14:13:19 | result |
 | xml.js:23:18:23:25 | source() | xml.js:20:14:20:17 | attr |
+| xml.js:26:27:26:34 | source() | xml.js:26:10:26:39 | convert ... (), {}) |

javascript/ql/test/library-tests/TaintTracking/xml.js

@@ -22,4 +22,7 @@
     
     parser.write(source()).close();
 
+    var convert = require('xml-js');
+    sink(convert.xml2json(source(), {})); // NOT OK
+
 })();