Clippy warning on beta channel for optional parameters

With the following code:

```
#![warn(clippy::ref_option)]

#[mockall::automock]
pub trait Store {
    fn find(&self, name: Option<String>) -> bool;
}
```

Running `cargo +beta clippy` generates a warning against the generated mock code:

```
warning: it is more idiomatic to use `Option<&T>` instead of `&Option<T>`
```