refresh table cell reference on reflow

Found a bug while using this with Angular. Angular destroyed the table header and re-rendered it; however floatthead kept a reference to the old table header and didn't realize that those elements no longer existed in the DOM. Because the number of columns didn't change, the call to `columnNum()` the table cell reference was never refreshed and floatthead failed to render the table properly.

This fix forces floatthead to fetch a new reference each time reflow is called.