Consistency with allow_none with serialization and deserialization

[matejsp]

I saw already comments how to workaround this issue:
#229 (comment)
#229 (comment)

But still I would like to marshmallow to be consistent with itself.

Basically you have a schema with allow_none=False and after dumping it you cannot load it back (without manually fixing the output that is not correct according to schema).

import marshmallow

class TransferSchema(marshmallow.Schema):
    id = marshmallow.fields.Int(allow_none=False)
    str = marshmallow.fields.Str(allow_none=False)


x = TransferSchema()
dumped = x.dump({
    'id': None,
    'str': None,
})

print(dumped)
loaded = x.load(dumped)
print(loaded)

error:

marshmallow.exceptions.ValidationError: {'id': ['Field may not be null.'], 'str': ['Field may not be null.']}

I think allow_none should be respected for both on serialize and deserialize.