refactor: avoid unwrap examples

[0xPoe]

close #3984

Just avoid unwrap as much as possible. Use expect to assert that it will always succeed.

[0xPoe]

We can also do:

                let mut response = Response::new(full(MISSING));
                *response.status_mut() = StatusCode::UNPROCESSABLE_ENTITY;
                return Ok(response);

I am not sure which one is actually better.