fix for #177

[Juravenator]

This implements a variation of the fix for #177 made by @Desproposito

[keanulee]

Thanks for the fix @Juravenator . Can you add a test so that we don't regress this?

[Juravenator]

I've been playing with the testing code to simulate a dragging gesture, but I can't seem to get it working.
I added an extra slider, since the test requires an initial value to be set using an attribute, and only an attribute.

<test-fixture id="customSlider">
    <template>
      <paper-slider value="50"></paper-slider>
    </template>
  </test-fixture>

I tried this approach to simulate a drag:

test('value doesn\'t max out on drag when initial value set', function() {
  // var node = customSlider.root.querySelector("paper-progress");
  var xy_middle = MockInteractions.middleOfNode(customSlider);
  // xy_middle.x += 10;
  // xy_middle.y += 10;
  var xy_end = {
    x: 0,
    y: xy_middle.y
  }
  MockInteractions.move(customSlider, xy_middle, xy_end);
});

As you can see, I tried modifying the paper-slider as well as the internal paper-progress that seems to be spawning the real events (observed using MonitorEvents($0) in chrome).

I seem to be missing something to make this work. Any suggestions?

[keanulee]

Bug fixed by #194