Fixed evaluating checkbox dirty status
    
    Problem:
    When value in checkbox is modified twice in a row (so it is at its original value) an 'undo' button is still visible even when it shouldn't be.
    
    Cause:
    IPA server sends boolean values as 'TRUE' or 'FALSE' (strings). Checkbox_widget converts them to JavaScript? boolean (true, false). Save method in checkbox_widget is returning array with a boolean. So test_dirty method always evaluates to dirty because 'FALSE' != false.
    
    This patch is fixing the problem.
    
    https://fedorahosted.org/freeipa/ticket/2494