Adding test case for FollowUpIds

The test logic:
1. creating a ticket and closing it (so we can later create a followUp ticket for it).
2. creating the followUp ticket, by setting the ViaFollowUpSourceId to the original closed ticket id.
3. Making sure that we can get the FollowUpIds of the original ticket.

Author: yuvalon

test/ZendeskApi_v2.Test/TicketTestPart2.cs

@@ -136,5 +136,37 @@ public async Task CustomDropDownFieldSaveAsync()
 
             Assert.True(api.Tickets.Delete(newTicket.Id.Value));
         }
+        
+        [Test]
+        public async Task CanGetFollowUpIds()
+        {
+            var ticket = new Ticket { Comment = new Comment { Body = "Original ticket", Public = false } };
+
+            var resp1 = await api.Tickets.CreateTicketAsync(ticket);
+
+            var closedTicket = resp1.Ticket;
+
+            closedTicket.Status = TicketStatus.Closed;
+
+            await api.Tickets.UpdateTicketAsync(closedTicket, new Comment { Body = "Closing Original Ticket" });
+
+            var ticket_Followup = new Ticket()
+            {
+                Subject = "I am the follow up Ticket",
+                Comment = new Comment { Body = "I will be linked to the closed ticket" },
+                Priority = TicketPriorities.Low,
+                ViaFollowupSourceId = closedTicket.Id.Value
+            };
+
+            var resp3 = await api.Tickets.CreateTicketAsync(ticket_Followup);
+            var resp4 = api.Tickets.GetTicket(closedTicket.Id.Value);
+
+            Assert.That(resp3.Ticket.Via.Source.Rel, Is.EqualTo("follow_up"));
+            Assert.AreEqual(resp4.Ticket.FollowUpIds.Count, 1);
+            Assert.AreEqual(resp4.Ticket.FollowUpIds.ElementAt(0), resp3.Ticket.Id);
+
+            Assert.That(await api.Tickets.DeleteAsync(resp3.Ticket.Id.Value), Is.True);
+            Assert.That(await api.Tickets.DeleteAsync(closedTicket.Id.Value), Is.True);
+        }
     }
 }