Abort previous pending XHR when navigating

example/page3.html

@@ -0,0 +1,15 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset='utf-8'>
+    <title>Hello</title>
+    <script src='../pjax.js'></script>
+    <script src='example.js'></script>
+  </head>
+  <body>
+    <div class='body'>
+      <h1>Page 3</h1>
+      Hello. Go to <a href='index.html' class="js-Pjax">Index</a>.
+    </div>
+  </body>
+</html>