Issue #2802403 by _Archy_, Berdir, rjay, catch, Tachion, jacktonkin,...

Issue #2802403 by _Archy_, Berdir, rjay, catch, Tachion, jacktonkin, oriol_e9g, willwh, tameeshb, dawehner, dhansen, bmcclure, DeFr, david.gil, gagarine, plach: Combination of language negotiation and path aliasing can cause a corrupted route cache, 404s

(cherry picked from commit c13d37fd)