Performance: Read the style.width property for the restore width and
apply that.

- This is much faster than the previous method of using $(this).width()
  since there is no longer an invalidation and getComputedStyle
  calculation. It just uses the value that is available in style.width,
  which might very well be empty (if the style attribute is used with a
  width property defined is it not empty).

- This also improves accuracy since it is the original that is restored,
  and not the calculated size.

- See http://datatables.net/forums/discussion/14811 for the discussion
  for this change.

- Thanks to `krzycho` for the discussion and suggestions.