get_mon_arrivals doesn't show accurate values

```
​t0 <- create_trajectory() %>%
    timeout(3)

env0 <- simmer() %>%
    add_generator("entity", t0, at(0)) %>%
    run()

get_mon_arrivals(env0)

     name start_time end_time activity_time finished
1 entity0          0        0             0     TRUE

```

Here the `end_time` of `entity0` should be `3`

Test with at least commit a09b025905954c464c80903e2b66378c79d54cbc 
