fix: auto now fields should be skipped

[mattwang44]

Describe the change

• Issue: When baker.make with _fill_optional set as True, the values of those DateTimeField with auto_now_add=True or auto_now=True from the resulting instance are different from the value stored in DB.
  • The test case from the first commit is to reproduce the issue

    
• solution: skip the auto datetime fields

PR Checklist

• Change is covered with tests
• CHANGELOG.md is updated if needed

[mattwang44]

could possibly resolve #469

[amureki]

@mattwang44 thank you for your first contribution! I am happy that you managed to figure out the issue and find your way through the code. 👍