Fix XUnit output with Gecko

the toString() function is not called by gecko because a DOMNode
does not inherits from Object. We should call a specific function
to serialize the content of the XML.

@@ -1619,7 +1619,7 @@ Tester.prototype.saveResults = function saveResults(filepath) {
     var exporter = require('xunit').create();
     exporter.setResults(this.suiteResults);
     try {
-        fs.write(filepath, exporter.getXML(), 'w');
+        fs.write(filepath, exporter.getSerializedXML(), 'w');
         this.casper.echo(f('Result log stored in %s', filepath), 'INFO', 80);
     } catch (e) {
         this.casper.echo(f('Unable to write results to %s: %s', filepath, e), 'ERROR', 80);

@@ -82,10 +82,6 @@ function XUnitExporter() {
     "use strict";
     this.results = undefined;
     this._xml = utils.node('testsuites');
-    this._xml.toString = function toString() {
-        var serializer = new XMLSerializer();
-        return '<?xml version="1.0" encoding="UTF-8"?>' + serializer.serializeToString(this);
-    };
 }
 exports.XUnitExporter = XUnitExporter;
 
@@ -158,6 +154,16 @@ XUnitExporter.prototype.getXML = function getXML() {
 };
 
 /**
+ * Retrieves generated Xunit XML
+ *
+ * @return string
+ */
+XUnitExporter.prototype.getSerializedXML = function getSerializedXML(xml) {
+    var serializer = new XMLSerializer();
+    return '<?xml version="1.0" encoding="UTF-8"?>' + serializer.serializeToString(this.getXML());
+}
+
+/**
  * Sets test results.
  *
  * @param TestSuite  results