fix: OrderProposal Customer Rejection (#180)

Author: nickevansuk

OpenActive.Server.NET/CustomBookingEngine/CustomBookingEngine.cs

@@ -581,9 +581,9 @@ public async Task<ResponseContent> ProcessOrderProposalUpdate(string clientId, U
                 SimpleIdComponents sellerIdComponents = GetSimpleIdComponentsFromApiKey(sellerId);
                 SimpleIdComponents customerAccountIdComponents = GetCustomerAccountIdComponentsFromApiKey(customerAccountId);
 
-                if (orderProposal == null || orderProposal.GetType() != typeof(Order))
+                if (orderProposal == null || orderProposal.GetType() != typeof(OrderProposal))
                 {
-                    throw new OpenBookingException(new UnexpectedOrderTypeError(), "OrderProposal is required for Order Cancellation");
+                    throw new OpenBookingException(new UnexpectedOrderTypeError(), "OrderProposal type is required for OrderProposal Customer Rejection");
                 }
 
                 // Check for PatchContainsExcessiveProperties