fix(send-email/mailchimp): Fix sending body

[jnv]

Description

Mailchimp map for communication/send-email is currently broken, as message properties are directly attached directly to body. This results in blank email being sent.

Motivation and Context

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have read the CONTRIBUTING document.
• [x] I haven't repeated the code. (DRY)
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.