During check() make sure that $mirror is the same size.

Hi Brandon,

Thank you very much for the jquery-expandable plugin, I plan on using this on a project I've been working on.

I noticed when I was playing with the example.html in Firefox 19.0 (which natively allows textarea resizing) that after I had resized the text area the increasing and decreasing height matching didn't work as expected.  If I made the text area wider, then the expansion would be quite a bit more than was needed, if smaller, then it wouldn't expand enough (and text would be hidden).

So, I believe that I've updated the code to prevent this by making sure the $mirror element is the same size as the text area when the check() function is called.

Best,
Ryan

P.S. This is my first pull request so apologies if I did anything incorrectly here.