swappableMux is now public and Append() now updates Server.Handler

Co-authored-by: Roberto Abdelkader Martínez Pérez <robertomartinezp@gmail.com>

internal/server/user/server.go

@@ -2,9 +2,13 @@ package user
 
 import (
 	"net/http"
+
+	"github.com/BBVA/kapow/internal/server/user/mux"
 )
 
-var Server http.Server
+var Server = http.Server{
+	Handler: mux.New(),
+}
 
 func Run() {
 }