Previously, when searching for a match, matchErr would be erroneously set, and prevent middleware from running (no match == no middleware runs).
This fix clears matchErr before traversing the next subrouter in a multi-subrouter router.
CORSMethodMiddleware sets the Access-Control-Allow-Methods response header
on a request, by matching routes based only on paths. It also handles
OPTIONS requests, by settings Access-Control-Allow-Methods, and then
returning without calling the next HTTP handler.
