Fixed custom tweet message to make it work with scheduled posts.

[iamdharmesh]

Description of the Change

As reported in #263, Custom tweet messages don't remain persistent in posts. So, when we set a custom tweet message and schedule it for publish in the future, it just sends the generic tweet, but not the Tweet from custom tweet message. PR fixes this issue.

Closes #263

How to test the Change

1. Create a post
2. Set custom tweet message and save post as draft
3. Refresh edit post page and verify that custom tweet message remain saved
4. schedule post to be publish in future
5. Verify that posted tweet is as per the custom tweet message when the scheduled post is scheduled.

Changelog Entry

Fixed - The custom tweet message does not work with scheduled posts.

Credits

Props @GeoffLambert77 @iamdharmesh

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.

[Sidsector9]

LGTM, approving 👍