Should we return nil when finding a non-hash value down the path?

With the current version

```ruby
hash = {
  "foo" => {
    "wat" => :not_a_hash
    "bar" => {
      "baz" => :value
    }
  }
}

Fat.at(hash, "foo", "wat", "baz")
# => :not_a_hash
```

But this could lead to confusion when you expect to get what `"baz"` is holding.

Should we return `nil` instead?