Quantize incoming digitals

[lovelydinosaur]

DecimalField now quantizes incoming values, ensuring that the resulting value includes the correct representation.

Previous the value would be correct, but would not be associated with the precision used by the field, so eg...

>>> class ExampleSerializer(serializers.Serializer):
>>>     total_price = serializers.DecimalField(max_digits=5, decimal_places=2)
>>> serializer = ExampleSerializer(data={'total_price': '12'})
>>> assert serializers.is_valid()
>>> print serializer.validated_data['total_price']
12.00  # Now correctly represented. Previously would print 12.

Closes #4318.

[pySilver]

Thanks!

[lovelydinosaur]

👍