Access latest route pattern when mapping route errors (#2823)

[987Nabil]

@jdegoes When handling errors of Route.Unhandled, we accessed the current route pattern to decode it based on the incoming Request. When nesting, the route pattern of the handler changes, but the decoding was already constructed based on the pre-nesting route pattern.

I fixed this by changed the handler in Route.Handled to handler: Handler[Any, Nothing, RoutePattern[_], Handler[Env, Response, Request, Response]] and when calling toHandler, handing over the latest route pattern.

I considered instead manipulating the Request to remove the prefix when nesting, but decided that this is a bad idea, since the Request that the user might access would have a path, that does not reflect the path of the remote request.

fixes #2823
/claim #2823

[jdegoes]

Good to have this one in! Algebraic laws FTW. 😄