fix: Fix add_header's handling of dicts

[JohnnyDeuss]

Fixes

The docstring for Mail.add_header states that the headers can be a Header object or a dict, however, passing a dict breaks due to attempted if header.personalization is not None: check.

Checklist

• [x] I acknowledge that all my contributions will be made under the project's license
• [x] I have made a material change to the repo (functionality, testing, spelling, grammar)
• [ ] I have read the Contribution Guidelines and my PR follows them
• [x] I have titled the PR appropriately
• [ ] I have updated my branch with the main branch
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added the necessary documentation about the functionality in the appropriate .md file
• [ ] I have added inline documentation to the code I modified

If you have questions, please file a support ticket.