Make SetIfDifferent no longer check if a key is mutable.

This would cause issues when updating an installation. By forcing removal of a key using an operation, we no longer check for children who override `IsMutableForKey`.

Fixes #40, and probably several others.

Author: richardjrossiii

Parse/ParseObject.cs

@@ -1134,7 +1134,7 @@ internal void SetIfDifferent<T>(string key, T value) {
       bool hasCurrent = TryGetValue<T>(key, out current);
       if (value == null) {
         if (hasCurrent) {
-          Remove(key);
+          PerformOperation(key, ParseDeleteOperation.Instance);
         }
         return;
       }