Correctly add terms to posts generated by Watson content classifiers

[ravinderk]

Description of the Change

I found that we overwrite terms when adding new terms to a post. Admin can set same or different WordPress taxonomy for Watson Category, Keyword , Entity & Concept. When the admin sets the same category for two or more Watson content classifiers, the term does not add correctly to the post. I update the logic to handle this use case.

Closes #340

How to test the Change

• Add the same taxonomy to all Watson content classifiers in ClassifAI > Language Processing and the new post. The post should have the correct terms.
• Add the different or same taxonomy to all Watson content classifiers in ClassifAI > Language Processing and the new post. The post should have the correct terms.

Changelog Entry

Fixed - Correctly add terms to posts generated by Watson content classifiers

Credits

Props @iamdharmesh , @ravinderk

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[ravinderk]

@dkotter Pull request is ready for code review.