Use all reserved predicates in IsReservedPredicateChanged. (#3531)

The method was not getting all the reserved predicates, making it return
true when a schema with ACL predicates was passed to an instance of
Dgraph with the ACL feature disabled.

schema/schema.go

@@ -473,7 +473,7 @@ func IsReservedPredicateChanged(pred string, update *pb.SchemaUpdate) bool {
 		return false
 	}
 
-	initialSchema := InitialSchema()
+	initialSchema := CompleteInitialSchema()
 	for _, original := range initialSchema {
 		if original.Predicate != pred {
 			continue