Horizontal Scroll Issue

Fixed an issue so the cloned header follows the original header if it is scrolled left / right.

Here a screenshot of the issue:

![image](https://f.cloud.github.com/assets/3167166/489570/cf643a7c-b99f-11e2-9bc6-61b2e6f79814.png)
<b>Example file:</b> <a href="https://github.com/zo103181/table-fixed-header/blob/master/horizontal-scroll-issue.html">horizontal-scroll-issue.html</a>

And the solution screenshot:

![image](https://f.cloud.github.com/assets/3167166/489583/3847216c-b9a0-11e2-814b-7c662c019404.png)
<b>Example file:</b> <a href="https://github.com/zo103181/table-fixed-header/blob/master/horizontal-scroll-issue%20(fixed).html">horizontal-scroll-issue (fixed).html</a>