fix(gateway): ensure directory listings have Content-Type text/html

Files already have an explicit Content-Type set. Be sure to do this for
directory listings as well to avoid a fallback to autodetection in
net/http. That fallback fails when a ResponseWriter is installed that
performs compression.

core/corehttp/gateway_handler.go

@@ -286,6 +286,7 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 		return
 	}
 
+	w.Header().Set("Content-Type", "text/html")
 	if r.Method == http.MethodHead {
 		return
 	}