Be tolerant of the the end key not existing in a /messages resposne (#489)

As spec'd:

> If no further events are available (either because we have reached the start of the timeline, or because the user does not have permission to see any more events), this property is omitted from the response.
>
> *-- https://spec.matrix.org/v1.1/client-server-api/#get_matrixclientv3roomsroomidmessages*

Split out from #214

Previously, it would throw the following error and potentially we found all of the events within that last response to satisfy everything,
```
JSONFieldStr: key 'end' missing
```

Author: MadLittleMods

tests/msc2716_test.go

@@ -1254,7 +1254,6 @@ func paginateUntilMessageCheckOff(t *testing.T, c *client.CSAPI, roomID string,
 		}))
 		callCounter++
 		messsageResBody := client.ParseJSON(t, messagesRes)
-		messageResEnd = client.GetJSONFieldStr(t, messsageResBody, "end")
 		// Since the original body can only be read once, create a new one from the body bytes we just read
 		messagesRes.Body = ioutil.NopCloser(bytes.NewBuffer(messsageResBody))
 
@@ -1294,6 +1293,12 @@ func paginateUntilMessageCheckOff(t *testing.T, c *client.CSAPI, roomID string,
 		if len(workingExpectedEventIDMap) == 0 {
 			return
 		}
+
+		// Since this will throw an error if they key does not exist, do this at the end of
+		// the loop. It's a valid scenario to be at the end of the room and have no more to
+		// paginate so we want to make sure the `return` above runs when we've found all of
+		// the expected events.
+		messageResEnd = client.GetJSONFieldStr(t, messsageResBody, "end")
 	}
 }