Multiselects with 0 in the values array auto-select 0 by default

### 1. Reproduction Steps

Add a new multiselect field to a LLMS settings section, like:

```
function test_course_lesson_metabox( $fields ) {

$values = array(
array(
'key' => '1',
'title' => '1',
),
array(
'key' => '2',
'title' => '2',
),
array(
'key' => 'Three',
'title' => 'Three',
),
array(
'key' => '0',
'title' => '0',
)
);

$selected = array( '1', 'Three' );

$fields['test'] = array( 'title' => 'Test', 'fields' => array() );

$fields['test']['fields'][] = array(
'id' => 'test',
'label' => 'Test',
'multi' => '1',
'type' => 'select',
'value' => $values,
'selected' => $selected
);

return $fields;

}

add_filter( 'llms_metabox_fields_lifterlms_course_options', 'test_course_lesson_metabox' );

```

### 2. Expected Behavior

+ The multiselect should show selected values of "1" and "Three"

### 3. Actual Behavior

+ Multiselect shows selected values of "0", "1", and "Three"

### 4. Solution
+ In llms.class.meta.box.select.php at line 67, change `if ( in_array( $option['key'], $selected ) ) {` to `if ( in_array( $option['key'], $selected, true ) ) {` so numbers aren't compared with strings.