Fix warning

Author: takezoe

build.sbt

@@ -3,3 +3,4 @@ organization := "io.github.gitbucket"
 version := "1.10.0"
 scalaVersion := "2.13.3"
 gitbucketVersion := "4.35.0"
+scalacOptions := Seq("-deprecation", "-language:postfixOps", "-opt:l:method", "-feature")

src/main/scala/gitbucket/notifications/controller/NotificationsController.scala

@@ -47,7 +47,7 @@ trait NotificationsControllerBase extends ControllerBase {
     val userName = params("userName")
     val disable = params.getAs[Boolean]("disable").getOrElse(false)
     updateEmailNotification(userName, disable)
-    flash += "info" -> "Notification setting has been updated."
+    flash.update("info", "Notification setting has been updated.")
     redirect(s"/${userName}/_notifications")
   })