fix: head changes should always be sent as an array

[lemmih]

Summary of changes

Changes introduced in this pull request:

• Send head changes as an array.

Unfortunately, we don't have a way of testing the chain_notify channel yet. That will be part of a later PR.

Reference issue to close (if applicable)

Closes #4069

Other information and links

Change checklist

• I have performed a self-review of my own code,
• I have made corresponding changes to the documentation,
• I have added tests that prove my fix is effective or that my feature works (if possible),
• I have made sure the CHANGELOG is up-to-date. All user-facing changes should be reflected in this document.

[LesnyRumcajs]

Is this covered by tests to ensure eternal compatibility?

[lemmih]

Is this covered by tests to ensure eternal compatibility?

Not yet. We don't have the infrastructure to test channel responses. :(

[lemmih]

Right now, we can only test request/response pairs. Channels and subscriptions require re-working our testing harness.