Decode cache data to the correct endpoint type

Use the return annotation to decode cached data to the correct type.
This follows the same logic FastAPI uses to JSON request bodies.

For the PickleCoder, this is a no-op as pickle already stores type
information in the serialised data.

tests/test_decorator.py

@@ -79,3 +79,10 @@ def test_method() -> None:
     with TestClient(app) as client:
         response = client.get("/method")
         assert response.json() == 17
+
+
+def test_pydantic_model() -> None:
+    with TestClient(app) as client:
+        r1 = client.get("/pydantic_instance").json()
+        r2 = client.get("/pydantic_instance").json()
+        assert r1 == r2