Fix nesting of opaque services that paths that contain params

[davidpdrsn]

While working on #830 I discovered that nesting opaque services at paths that contain params was totally broken. Things like

let api_routes = Router::new().route("/:b", get(|| async {})).boxed_clone();
let app = Router::new().nest("/:a", api_routes);

We would try and strip the prefix as /:a but thats a placeholder that should match anything. It shouldn't be used as the literal prefix.

This required some changes to StripPrefix. I also added a bunch more tests and some property tests to get better coverage against panics.

[davidpdrsn]

Honestly surprised no one ran into this bug before. I guess users are either nesting ServeDir or Router.

[davidpdrsn]

I swear I had the tests passing locally but I must have fat fingered something. I'll get it fixed.