py3: send Decimal number as string instead of base64 encoded value
    
    for Decimal only from client to server direction uses __base64__
    notation. Server replies with pure string for Decimal data, and also
    server is able to parse string and create decimal values where needed.
    
    without this we need ugly py3 code:
    -        return {'__base64__': base64.b64encode(str(val))}
    +        return {'__base64__': base64.b64encode(
    +            str(val).encode('ascii')).decode('ascii')}
    
    https://fedorahosted.org/freeipa/ticket/4985
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>