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.

[seanmonstar]

Thanks for cleaning these up!

Just one general comment that applies to several of these: it's never the body that would cause an error. It's only every status(), header(), or uri() that is parsing into an http type that could error. So the expect should mention "constant status won't error", instead of "missing should be valid".

[0xPoe]

It's only every status(), header(), or uri() that is parsing into an http type that could error. So the expect should mention "constant status won't error", instead of "missing should be valid".

Good point! Updated. Thank you.