Document._class_id is always overwritten

[volfpeter]

Describe the bug

Document._class_id is overwritten. Document initialization should check if it's set before assigning a value. This messes up the customization of document discriminators and leads to failing queries (update, find, link fetching...).

To Reproduce

Create a couple of documents (with inheritance pattern), set Settings.class_id for example to "type", add type: str = "my-doc-type-1", type: str = "my-doc-type-2", ... properties to documents and also set _class_id on each document to the same value as the type property.

Expected behavior

Update, find, link fetching queries work.

Additional context

I'll submit a PR with a fix that avoids overwriting _class_id if it's already set. It solves the issues.