_feature_sanitize is inconsistent when the array is associative with only one entry

This error was highlighted in the features_test.module and features.test.

The default field setting was

'settings' => array(
        'text_processing' => '0',
),

The Normal field setting was

'settings' => array(
        'text_processing' => '0',
        'user_register_form' => false,
),

Default was sanitized to

'settings' => array(
    0 => '0',
),

While normal was sanitized to

'settings' => array(),

Sanitization needs some work....