Make sure that all endpoints that accept request body parameters can only receive an object

The latest version of the OpenAPI spec allows the request body to be set to an array of strings in some cases (e.g. `POST /repos/{owner}/{repo}/issues/{issue_number}/labels`). These are no longer documented as far as I know and are there for legacy reasons, but having them be part of the OpenAPI spec makes it hard to generate usable SDKs / types from it.