fixing .chosen-select height bug on webkit

The opaque `.chosen-select` node was not exactly the same height as the
`.chosen-container` node behind it, which allowed the possibility of
some tap events on the edges to make it down to the other chosen nodes,
triggering the non-native behavior. This change should prevent that from
happening.

Webkit will not respect the `height` style for `select` nodes with the
default `-webkit-appearance: menulist` style. We can change this by
changing `-webkit-appearance` to `menulist-button`, which does respect
the `height` style.

sass/chosen.scss

@@ -36,6 +36,7 @@ $chosen-sprite-retina: image-url('chosen-sprite@2x.png') !default;
     display: none;
   }
   &.chosen-use-native select {
+    -webkit-appearance: menulist-button;
     display: initial;
     height: 100%;
     opacity: 0;