Fixed metadata serialization of Numbers and DNs
    
    There were following problems:
    1. DNs and Decimals weren't properly serialized. Serialization output was object with empty __base64__ attribute. It was fixed by converting them to string.
    2. numberical values equal to 0 were excluded from metadata. It broke many of minvalue checks in Web UI. Now excluding only None and False values as initally intended.
    
    https://fedorahosted.org/freeipa/ticket/3052