Do not overwrite creating fields if were set explicitly

[st4lk]

Sometimes, I'd like to specify different user for CreatingUserField field by myself. Such cases are rare, usually I'm ok with populating this field from request.user automatically. But they still happens.

Example:

from audit_log.models.fields import CreatingUserField

class MyModel(models.Model):

    created_by = CreatingUserField(...)

# --------------------

def my_view(request):

    if request.method == 'POST':
        # created_by will be filled from request.user, that is ok
        MyModel.objects.create()

        # but here I'd like to change the created_by user.
        # I can't do this because it will be always
        # equal to request.user. Since I've specified the user explicitly - audit_log
        # should assume that I know what I'm doing.
        another_user = User.objects.get(id=123)
        MyModel.objects.create(created_by=another_user)

Solution: fill CreatingUserField automatically only if no value was provided. Otherwise - don't overwrite it.

This PR miss the tests. I'll add them if such behaviour will be approved.