tables/fix 6255: getAttribute colspan vs. data-colspan

@gabrielschulhof, @uGoMobi, @MythosVylok:

Fix for #6255. 

Setting `true` in **table.js** on this line:
````
var span = parseInt( $.mobile.getAttribute( this, "colspan" ), 10 ),
````
will try to get `data-colspan` vs. `colspan` on the header rows, which returns NaN, so therefore only the first grouped row was in `$cells`. Removing `true` fixes this. 

Demo: [here](http://www.franckreich.de/jqm/bugs/6255/index.html)