contrib/net/http: NewServeMux does not respect WithResourceNamer option

[mccutchen]

The NewServeMux wrapper in the contrib/net/http package takes a slice of Options, but if the WithResoureNamer option is given it is ignored and a hard coded resource name is always used instead:

dd-trace-go/contrib/net/http/http.go

Lines 47 to 53 in 50bd7ce

	resource := r.Method + " " + route
	TraceAndServe(mux.ServeMux, w, r, &ServeConfig{
	Service: mux.cfg.serviceName,
	Resource: resource,
	SpanOpts: mux.cfg.spanOpts,
	Route: route,
	})

[katiehockman]

Thanks for filing the issue and for the PR 👍