Ignoring none field on serialization if it is not required.

[thgbarros]

If the class had a default value defined with none, the serializer class was serialize the field with
{field: None}.

class ASerializer(Serializer):
   a = Field(required=False)   

class AObject:
   def __init__(a = None):
     self.a = a

print(ASerializer(AObject()).data) 
output = {'a': None} 

Now:

class ASerializer(Serializer):
   a = Field(required=False)

class AObject:
   def __init__(a = None):
     self.a = a

print(ASerializer(AObject()).data) 
output = {}  

but if the field is required, the output have a field none.

class ASerializer(Serializer):
   a = Field()

class AObject:
   def __init__(a = None):
     self.a = a

print(ASerializer(AObject()).data) 
output = {'a': None}  

[coveralls]

Coverage decreased (-0.8%) to 99.174% when pulling db79da7 on thgbarros:master into 56e5a27 on clarkduvall:master.

[coveralls]

Coverage decreased (-0.8%) to 99.174% when pulling db79da7 on thgbarros:master into 56e5a27 on clarkduvall:master.

[clarkduvall]

This is actually the expected behavior, for more info you can check out #56 and the fix in #57. To have the field removed from the output, it should not be present at all in the input.

Thanks!