Update module gorilla/mux to v1.8.0

go.mod

go.sum

vendor/github.com/gorilla/mux/.travis.yml

vendor/github.com/gorilla/mux/ISSUE_TEMPLATE.md

vendor/github.com/gorilla/mux/README.md

vendor/github.com/gorilla/mux/context.go

vendor/github.com/gorilla/mux/doc.go

vendor/github.com/gorilla/mux/go.mod

vendor/github.com/gorilla/mux/middleware.go

vendor/github.com/gorilla/mux/mux.go

vendor/github.com/gorilla/mux/regexp.go

vendor/github.com/gorilla/mux/route.go

vendor/github.com/gorilla/mux/test_helpers.go

vendor/modules.txt