v4 API: Don't map Products

[n7studios]

Summary

We don't need to map returned Products into an associative array, as this is already done in the API call.

Testing

• testRefreshProducts: Test that calling refresh on Products returns and caches Products correctly.
• testRefreshPosts: Test that calling refresh on Posts returns and caches Posts correctly.

Checklist

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)