`assert_covers` treats arrays as maps

`assert_covers` should treat arrays as scalars, i.e. never assume that one array may cover another unless they are equal. For example, the following assertions must fail while currently they pass:

```lua
t.assert_covers({1, 2, 3}, {1, 2})
t.assert_covers({a = {1, 2, 3}}, {a = {1, 2}})
```