[3818] Zwiększenie pokrycia testami.

https://gist.github.com/lukkolstx/bd8e4a7df17657f4a60d
First of all variables (arguments) should speak for themselves - `status_code` and `content_type` would be fine.
Second, many of those values are repeated (always `200`, `'application/json'`) so you could provide them as default argument values or set them permanently.