Issues with example code

Hi, I'm trying the example code, and I stumbled upon [this](https://github.com/async-email/async-imap/blob/master/tests/imap_integration.rs#L173) specific line.

When reproducing it, I had to add a `.as_ref()` to make the compiler stop complaining. Also an extra `unwrap()`. As so:

```
let e = fetch.as_ref().unwrap().envelope().unwrap();
```

I don't see the CI failing. So I'm wondering if the CI never ends up running building this line of code or whether I'm doing something wrong.

I am 0.4.1.

Thanks for all the work.

Start at tests/imap_integration.rs#L173 and compare the example with the async-imap 0.4.1 behavior described in the issue. Run the relevant build or CI test to determine whether this line is compiled. Done means the example and its CI coverage agree, or the required version-specific behavior is documented.

Written by the indexing model from the issue text.