Update MY_Model.php

I need update_where with multiple field condition ex: 
```UPDATE employee SET division = 1 WHERE employee_role = 1 AND year  = 2013```
rather than extending the method, I modified the update_where
```
public function update_where($field=NULL, $value=NULL, $data=NULL)
{
  return $this->update(array($field => $value), $data);
  $where = is_array($field) ? $field : array($field => $value);
  return $this->update($where, $data);
}
```
`so the usage is something like this
```
$data['division'] = 1;
$where[employee_role'] = 1;
$where['year'] = 2013;
$this->update_where($where,null,$data);
```