Apply defaults to fields with None value at 'set' time.

If a field has a default, and you explicitly set it to None, the behaviour before this patch was very confusing:

    class Person(Document):
        created = DateTimeField(default=datetime.datetime.utcnow)

    >>> p = Person(created=None)
    >>> p.created
    datetime.datetime(2013, 5, 30, 0, 18, 20, 242628)
    >>> p.created
    datetime.datetime(2013, 5, 30, 0, 18, 20, 995248)
    >>> p.created
    datetime.datetime(2013, 5, 30, 0, 18, 21, 370578)

It would be stored as None, and then at 'get' time, the default would be applied. As you can see, if the default is a generator, this leads to some crazy behaviour.

There's an argument that if I asked it to be set to None, why not respect that? But I don't think that's how the rest of mongoengine seems to work (for example, setting a field to None seems to mean it doesn't even get set in mongo - as opposed to being set but with a 'null' value). Besides, as the code shows above, you'd expect p.created to return None. So clearly, mongoengine is already expecting None to mean 'default' where a default is available.

This bug also interacts nastily with required=True - if you're forcibly setting the field to None, then at validation time, the None will fail validation despite a perfectly valid default being available.

With this patch, when the field is set, the default is immediately applied. This means any generation happens once, the getter always returns the same value, and 'required' validation always respects the default.

Note: this breakage seems to be new since mongoengine 0.8.