form_dropdown array as first param fix

Form_dropdown was previously incompatible with most other form_ helper methods. This patch fixes that and now allows arrays to be passed in just as the other functions support:
```
$type_ids = array(1,2,3,4,5)
$dropdown = array(
            'name' => 'product[type_id]',
            'id' => 'product_type_id',
            'class' => 'form-control',
            'selected' => $this->form_validation->set_value('product[type_id]', $product->type_id),
            'options' => $type_ids,
);

echo form_dropdown($dropdown);
```

Will produce:
```
<select name="product[type_id]" id="product_type_id" class="form-control">
    <option value="1">1</option>
    <option value="2">2</option>
    <option value="3">3</option>
    <option value="4">4</option>
    <option value="5">5</option>
</select>
```