actix_router: Leaking strings causes LeakSanitizer noise

`ResourceDef` leaks strings [on purpose](https://github.com/actix/actix-net/blob/router-v0.2.7/actix-router/src/resource.rs#L63), which makes LeakSanitizer output rather messy, since the strings aren't explicitly free'd. I understand why this is (and why it's not a problem for execution), but this makes using LeakSanitizer rather ... unwieldy.

I'm also unsure what could be done here - dropping strings when the ResourceDef is dropped isn't exactly safe, and using `Rc` instead would (I guess) be a perf hit.