Firefox on Windows triggers the click event for the right mouse button

Unfortunately I am having trouble to execute `gulp test` in windows. Several tests fail. I suppose this is going to be properly addressed once #52 is landed.

For that reason I wasn't able to create a proper test case for this pull, but the test should be pretty straightforward:

```javascript
var element = document.querySelector('a[href="/base/page#hash"]');
// https://developer.mozilla.org/en-US/docs/Web/API/event.button
element.dispatchEvent(new MouseEvent('click', {button: 2}));
assert.strictEqual(app.pendingNavigate, null);
done();
```