`AsyncFilter::layer` returns `FilterLayer`

[davidpdrsn]

This seems to be a copy/paste mistake. AsyncFilter::layer returns FilterLayer, its meant to return AsyncFilterLayer. See https://docs.rs/tower/0.4.8/tower/filter/struct.AsyncFilter.html#method.layer

I wonder if fixing this would be considered a breaking change 🤔

[hawkw]

Aghh! This is definitely my fault, whoops.

This is technically a breaking change, but I think it might be semver-compliant to fix it in a minor version, since the previous code was incorrect, and if anyone was using it, they weren't getting what they probably expected to get. I'd want to get @carllerche's opinion on the semver part, though.

With that said, it's probably fine to just publish a new version, too...