Fix default field when is a simple callable function

When I have::

    from time import time
    
    def utc_now_miliseconds():
        return (int(time() * 1000))
    
    class HouseSerializer(serializers.Serializer):
        name = serializers.CharField(max_length=200)
        timestamp = serializers.IntegerField(default=utcnow_miliseconds)
   

If I consume an endpoint with this serializer the function return the value::

    curl -XPOST -d "name=23" http://localhost:8000/api/home/

the values are name=23 and timestamp is some 1369846517231