add default value to model serializer

[duduklein]

Hi, as I explained in #2683, the default value is not being passed in the model serializer.

I understand your argument that this is not required functionality-wise, but it is a problem when auto generation documentation (e.g. with django-rest-swagger). This was the behavior in djangorestframework==2.3.8, in which the swagger tests are run against.

I don't see any disadvantage in passing the default value to the field, instead of just marking it as non required.

Do you see any?
Please reconsider.

[tomchristie]

I don't see any disadvantage in passing the default value to the field, instead of just marking it as non required.

This would be a big change in behavior. The pull request as it stands also wouldn't be correct, as it'd generate the default value once and once only, even for callables, such as current time/date.