Incorporate changes from parse-community#1589

Author: eeallen1

Parse/Parse/PFObject.m

@@ -207,7 +207,7 @@ + (BFTask *)_enqueue:(BFTask *(^)(BFTask *toAwait))taskStart forObjects:(NSArray
 + (BOOL)collectDirtyChildren:(id)node
                     children:(NSMutableSet *)dirtyChildren
                        files:(NSMutableSet *)dirtyFiles
-                        seen:(NSSet *)seen
+                        seen:(NSMutableSet *)seen
                      seenNew:(NSSet *)seenNew
                  currentUser:(PFUser *)currentUser
                        error:(NSError * __autoreleasing *)error {
@@ -288,7 +288,7 @@ + (BOOL)collectDirtyChildren:(id)node
             if ([seen containsObject:object]) {
                 return YES;
             }
-            seen = [seen setByAddingObject:object];
+            [seen addObject:object];
 
             // Recurse into this object's children looking for dirty children.
             // We only need to look at the child object's current estimated data,
@@ -330,7 +330,7 @@ + (BOOL)collectDirtyChildren:(id)child
     return [self collectDirtyChildren:child
                              children:dirtyChildren
                                 files:dirtyFiles
-                                 seen:[NSSet set]
+                                 seen:[NSMutableSet set]
                               seenNew:[NSSet set]
                           currentUser:currentUser
                                 error:error];
@@ -2805,4 +2805,4 @@ + (void)deleteAllInBackground:(NSArray<PFObject *> *)objects target:(nullable id
     }];
 }
 
-@end
+@end