count_query fails for models using an alternate database connection

If an Eloquent model has

````
protected $connection='my_alternate_connection'
````

set, DataTable::getRows() will throw an error when the count_query tries to re-run the query using the default connection.
Merge branch 'patch-2' of github.com:claar/Laravel-Administrator into dev
Fixing a small error and updating the tests (Closes #247)