Core API: Newsletter required fields

When creating or updating newsletters, the Core API requires the “source” field, but it is optional for the Open API. I think the Core API should be updated to make it optional as well.

When updating a newsletter via the Core API (HTTP PUT), I am required to provide an email and a source, even when these values already exist on the object in Pelcro. The Core API should not require me to pass these values for records that already exist.