Reset the "fired unload" flag at the end of document.open()

Fixes https://www.w3.org/Bugs/Public/show_bug.cgi?id=28772.

Author: domenic

source

@@ -88181,6 +88181,9 @@ document.body.appendChild(frame)</pre>
    does not have a <code>Document</code> object, so a new one will be created if the session history
    is traversed to that entry.</p></li>
 
+   <li><p>Set the <code>Document</code>'s <span>fired unload</span> flag to false. (It could have
+   been set to true during the <span data-x="unload a document">unload</span> step above.)</p></li>
+
    <li><p>Finally, set the <span>insertion point</span> to point at just before the end of the
    <span>input stream</span> (which at this point will be empty).</p></li>