Returns the handle registered with the given path (key). The values of
wildcards are saved to a map.
If no handle can be found, a TSR (trailing slash redirect) recommendation is
made if a handle exists with an extra (without the) trailing slash for the
given path.
Params is a Param-slice, as returned by the router.
The slice is ordered, the first URL parameter is also the first slice value.
It is therefore safe to read values by the index.