Fieldmanager_Select option groups are incorrectly adding option values

When creating a Fieldmanager_Select instance using the grouped syntax, the array key is used for the option text and the array value is used for the option value. This is inconsistent from the ungrouped / documented usage.

**Fieldmanager_Select (ungrouped / documented usage)**

``` php
$fm = new Fieldmanager_Select( array(
'name' => 'test_select_ungrouped',
'options' => array(
'zero' => 'Option 0',
'one' => 'Option 1',
'two' => 'Option 2'
)
) );
$fm->add_meta_box( 'Select box (ungrouped)', 'page' );
```

Output (expected):

``` html

Option 0
Option 1
Option 2

```

**Fieldmanager_Select (grouped / undocumented)**

``` php
$fm = new Fieldmanager_Select( array(
'name' => 'test_select_grouped',
'grouped' => true,
'options' => array(
'Group 1' => array(
'group1_zero' => 'Option 0',
'group1_one' => 'Option 1',
'group1_two' => 'Option 2'
),
'Group 2' => array(
'group2_zero' => 'Option 0',
'group2_one' => 'Option 1',
'group2_two' => 'Option 2'
)
)
) );
$fm->add_meta_box( 'Select box (grouped)', 'page' );
```

Output

``` html

group1_zero
group1_one
group1_two


group2_zero
group2_one
group2_two

```