[116] Support Callable default and Always Use default If Present

[timjklein36]

What

This change fixes #116, the following bug:

If a field is of a class that does not have a generator (default or provided by the user) also has a default value which is a callable, the generated value is returned as the callable, not the result of calling the same.

For instance:

from django.db import models
from model_bakery import baker


class MyField(models.TextField):
    pass

class DummyModel(models.Model):
    test = MyField(default=lambda: "default value")


dummy = baker.make(DummyModel)

# Before fix
dummy.test  # <function <lambda> at 0x7f0eadb9dca0>

# After fix
dummy.test  # "default value"

This also addresses the fact that the default value for a field was not always being returned from Baker.generate_value since it was lower in the if, elif, else block which also checked for the generator function to use for that field's value.

Note: This PR takes the stance that: if default is present, that value should be used unless an explicit kwarg for that field is supplied to make.

Why

This will allow fields of classes that are unknown to model_bakery, yet have a desired default value, to be used without error.

[anapaulagomes]

LGTM! 👍🏽

[berinhard]

Thanks for your PR @timjklein36

[berinhard]

Thanks for this catch! I didn't notice that the assert was wrong before