Add rounded corners to slider range area

The gray area in a range slider should have rounded corners, too. Otherwise the rounded corners of the parent area are being overlapped (it's subtle on white background, I admit).

Before: https://dl.dropbox.com/u/1806977/jquery-ui-slider-before.png
After: https://dl.dropbox.com/u/1806977/jquery-ui-slider-after.png