Keep original style attr values when restoring original state

Currently, original style attribute values in the slider elements are removed when restoring the original state. This affects users who use the style attribute in their children/main slider element as the original value isn't kept when the plugin restores its state.

Original HTML:

```html
<ul>
<li style="background: blue"><a href="#">Link</a></li>
<li style="background: red"><a href="#">Link</a></li>
</ul>
```

After calling destroySlider (before fix):

```html
<ul>
<li><a href="#">Link</a></li>
<li><a href="#">Link</a></li>
</ul>
```

Changes done ensures that original styles are kept.

After calling destroySlider (after fix):
```html
<ul>
<li style="background: blue"><a href="#">Link</a></li>
<li style="background: red"><a href="#">Link</a></li>
</ul>
```