Don't wipe style attribute in foundation.dropdown.js#css

In dropdown.js, css function:

```javascript
dropdown.attr('style', '').css({
  position : 'absolute',
  top: position.top + this.outerHeight(target),
  left: left
});
```

By wiping the style attribute, it can needlessly break other plugins bound to the same element.

As a use case, I'm using foundation dropdown with Packery for a "meganav".