Fixed problem with double foreach on the same Iterator

Hi, I found hidden problem in the Entity source. 

```$columns``` variable is Iterator and first foreach on https://github.com/Abhoryo/APYDataGridBundle/blob/master/Grid/Source/Entity.php#L327 is using it.

Inside this foreach is a method call ```$this->getColumnById(...)```. This method coverts iterator to the array and accidentally reset its pointer for the first foreach.