Improve return values for zscan and hscan

Makes `#zscan` return an array of key/score pairs as its second return value, to more closely match the return value of `#zrange`:

```
r.zscan "scores", 0
# => [7, [["a", 2], ["d", 18.0], ["g", 32.0]]]
```

Makes `#hscan` return an array of key/value  pairs as its second return value:

```
r.hscan "details", 0
# => [0, [["name", "bob"], ["age", "42"]]]
```

As suggested by @ federicobond on https://github.com/redis/redis-rb/pull/379