fix envelope message from 'Message' object to raw bytes.

[marcofavorito]

Proposed changes

Fix the Envelope class such that the field message is bytes rather than Message.

Fixes

None.

Types of changes

What types of changes does your code introduce to agents-tac? Put an x in the boxes that apply

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

Checklist

Put an x in the boxes that apply.

• [X] I have read the CONTRIBUTING doc
• [X] I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• [ ] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)
• [ ] Any dependent changes have been merged and published in downstream modules

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...