Add missing tests

We don't yet really have enough tests to score even the minimum of 1 point on the IP Maturity Matrix:

> Specs which cover golden path / APIs have full OpenAPI Definition

Currently we have specs that cover the `AddContent` service collection extension method, and the `GetContent` service provider extension method. This is essentially just a way of constructing objects based on a content type (MIME) string instead of a type name. And there are also specs for testing JSON serialization and deserialization.

We currently have no tests for content handling which, as the name suggests, is this library's main job. We need to add these before we can reasonably say we cover the golden paths.