Newsletter signup: Mailchimp tags are not passed from Newspack to Mailchimp

[kcroygithub]

Describe the bug
In newsletter settings there is the option to attach Mailchimp Tags to a newsletter signup. These are important as they allow us to differentiate different types of signups, and receive different welcome email using Automations. I have selected a Tag in Newspack Newsletter settings, however those tags do not show up when the contact is viewed in Mailchimp. Speaking with Mailchimp support, I was told "The request to mailchip is passes along an object. No tag object is being sent along. The API is not passing along a request for tags"

To Reproduce

1. Wordpress Dashboard-> Newsletters -> Settings
2. Scroll down to subscription lists
3. Ensure Mailchimp Audience and Mailchimp Group are toggled on
4. Toggle on a Tag
5. Save Subscription list
6. Sign up for newsletter on the website
7. In Mailchimp find the new contact
8. Examine tags for new contact
9. No tags have been set

Expected behavior
Selecting a Tag in the subscription list should create a contact in Mailchimp that has the specified Tag added to it