Make PREVIOUS_GAME_MODE read-only

Author: Lignium

src/main/java/org/spongepowered/api/data/Keys.java

@@ -2182,7 +2182,7 @@ public final class Keys {
     public static final Key<Value<Integer>> POWER = Keys.key(ResourceKey.sponge("power"), Integer.class);
 
     /**
-     * The previous {@link GameMode} of a {@link ServerPlayer}.
+     * The previous {@link GameMode} of a {@link ServerPlayer}. Readonly.
      */
     public static final Key<Value<GameMode>> PREVIOUS_GAME_MODE = Keys.key(ResourceKey.sponge("previous_game_mode"), GameMode.class);
 

src/main/java/org/spongepowered/api/entity/living/player/server/ServerPlayer.java

@@ -349,8 +349,8 @@ default Value.Mutable<GameMode> gameMode() {
      *
      * @return The previous game mode of the player
      */
-    default Value.Mutable<GameMode> previousGameMode() {
-        return this.requireValue(Keys.PREVIOUS_GAME_MODE).asMutable();
+    default Value<GameMode> previousGameMode() {
+        return this.requireValue(Keys.PREVIOUS_GAME_MODE);
     }
 
     /**