Fixed the existing instance being deleted in Awake

Replaced `else` with `else if (instance != this)` as a condition for the instance to be deleted in Awake. This fixes a really weird bug where if singleton was used in the editor and then again in play mode, it would delete the original instance and create a new one. Closes #21

Author: nnra6864

Runtime/Scripts/MonoSingleton.cs

@@ -72,7 +72,7 @@ protected virtual void Awake()
  // Initialize existing instance
  InitializeSingleton();
  }
- else
+ else if (instance != this)
  {
 
  // Destory duplicates
@@ -147,4 +147,4 @@ public static void DestroyInstance()
 
  #endregion
  }
-}
+}