Filter::or with a filter's error of Never should also have Never as error

[seanmonstar]

If you chain 2 filters with an or, and the latter one can never fail, it is clear that the chained filter likewise can never fail. However, this is currently not true:

fn v1() -> impl Filter<Extract=(Option<u32>,), Error=Rejection> {
    warp::path::param::<u32>()
        .map(Some)
        .or(warp::any().map(|| None))
}

It should be like this:

fn v1() -> impl Filter<Extract=(Option<u32>,), Error=Never> {
    warp::path::param::<u32>()
        .map(Some)
        .or(warp::any().map(|| None))
}

I've made changes locally that makes this work, by modifying CombineRejection, but I suspect this change may subtly break some people...