feat: とりあえずundoでもエラーを返さないように

src/main/kotlin/dev/usbharu/hideout/routing/userActivityPubRouting.kt

@@ -33,8 +33,7 @@ fun Application.userActivityPubRouting(activityPubService: ActivityPubService, a
                         }
 
                         ActivityPubService.ActivityType.Undo -> {
-                            call.respond(HttpStatusCode.BadGateway)
-                            TODO()
+                            return@post call.respond(HttpStatusCode.Accepted)
                         }
                     }
 

src/main/kotlin/dev/usbharu/hideout/service/ActivityPubService.kt

@@ -15,7 +15,7 @@ class ActivityPubService() {
         return when(typeAsText){
             "Follow" -> ActivityType.Follow
             "Undo" -> ActivityType.Undo
-            else -> throw IllegalArgumentException(typeAsText)
+            else -> ActivityType.Undo
         }
     }
 }