Fix addattr internal error
    
    When ADD command is being executed and a single-value object attribute
    is being set with both option and addattr IPA ends up in an internal
    error.
    
    Make better value sanitizing job in this case and let IPA throw
    a user-friendly error. Unit test exercising this situation is added.
    
    https://fedorahosted.org/freeipa/ticket/2429