serpy no longer supports None as field values

[terite]

As a result of #38 serpy the following things happen when None is returned from a MethodField

• If required=True (default), serialization throws an exception
• If required=False, the key and value are not included in the output dictionary.

I'm using MethodField because it's a concrete example. I believe the same thing happens with most (if not all) other fields.

As you might expect, this breaks any use case of serpy that needs to return None.

The django rest framework docs state

Setting this to False also allows the object attribute or dictionary key to be omitted from output when serializing the instance. If the key is not present it will simply not be included in the output representation.

In concrete terms, the following serializer returns the data as seen below

class DrfSerializer(serializers.Serializer):
    value = serializers.IntegerField(required=...)

DRF 3.7.3	required=True	required=False
data = {'value': 1}	{'value': 1}	{'value': 1}
data = {'value': None}	{'value': None}	{'value': None}
data = {}	KeyError(u"Got KeyError when...	{}

When using the following serpy 0.2.0 serializer I get the results below

class SerpySerializer(serpy.DictSerializer):
    value = serpy.Field(required=...)

serpy 0.2.0	required=True	required=False
data = {'value': 1}	{'value': 1}	{'value': 1}
data = {'value': None}	TypeError('Field {0} is required', 'value')	{}
data = {}	KeyError('value',)	KeyError('value',)

For reference, this is the output from serpy 0.1.1

serpy 0.1.1	required=True	required=False
data = {'value': 1}	{'value': 1}	{'value': 1}
data = {'value': None}	{'value': None}	{'value': None}
data = {}	KeyError('value',)	KeyError('value',)

As seen in the tables, PR #38 does the following things

• breaks cases that previously matched DRF behavior
• adds no new cases that match DRF behavior
• breaks any serializers that wanted to return None.

For these reasons I'd like to suggest #38 be reverted.

[clarkduvall]

Released this change in version 0.3.0