Consistency with allow_none with serialization and deserialization

Nobody has claimed this yet.

I saw already comments how to workaround this issue:
https://github.com/marshmallow-code/marshmallow/issues/229#issuecomment-363250656
https://github.com/marshmallow-code/marshmallow/issues/229#issuecomment-134387999

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.

Start with the provided TransferSchema example and trace how allow_none is handled during dump and load for the Int and Str fields. Confirm the intended behavior against the linked issue discussions, then add or update coverage so serialization and deserialization treat null values consistently.

Written by the indexing model from the issue text.