Add support for message headers

[yannp]

Pull Request Template

Description

Original issue: https://github.com/flipp-oss/deimos/issues/192

Currently deimos doesn't support providing :headers to phobos when publishing a message. A monkeypatch prevents it.

Fixes #192 (issue)

Type of change

Please delete options that are not relevant.

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [X] This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Added new specs to cover passing down headers to phobos.

Checklist:

• [X] My code follows the style guidelines of this project
• [X] I have performed a self-review of my own code
• [X] I have commented my code, particularly in hard-to-understand areas
• [X] I have made corresponding changes to the documentation
• [X] I have added a line in the CHANGELOG describing this change, under the UNRELEASED heading
• [X] My changes generate no new warnings
• [X] I have added tests that prove my fix is effective or that my feature works
• [X] New and existing unit tests pass locally with my changes
• [X] Any dependent changes have been merged and published in downstream modules

[dorner]

Looks great! Thanks for the contribution!

[yannp]

@dorner I realise that with this PR I didn't address the DB backend properly. This would require to add a headers column to the kafka_messages table. That would be a major update as I understand it. What do you think?

[dorner]

Hmm... let's see if we can do it wrapped in some checks (respond_to?, checking columns etc.) so it wouldn't break backwards compatibility.