Fix build issue following latest AppVeyor update.

Author: KeRNeLith

tests/QuikGraph.Data.Tests/DataRelationEdgeTests.cs

@@ -90,12 +90,12 @@ void CreateDataColumns(out DataColumn custIdCol, out DataColumn ordIdCol)
 
                 var customers = new DataTable("Customers");
                 custIdCol = new DataColumn("CustomerID", typeof(int)) { Unique = true };
-                customers.Columns.Add(customerIdCol);
+                customers.Columns.Add(custIdCol);
                 dataSet.Tables.Add(customers);
 
                 var orders = new DataTable("Orders");
                 ordIdCol = new DataColumn("OrderID", typeof(int)) { Unique = true };
-                orders.Columns.Add(orderIdCol);
+                orders.Columns.Add(ordIdCol);
                 dataSet.Tables.Add(orders);
             }