Fix sorted set range argument

Fix the return value from was different when you specify the Range

```ruby
require "redis-objects"
require "redis/objects/version"
Redis::Objects::VERSION         # => "0.9.1"

class C
  include Redis::Objects
  sorted_set :v
  def id
    1
  end
end

r = C.new
r.v[:a] = 1
r.v[:b] = 1
r.v[:c] = 1
r.v.members[0...2]            # => ["a", "b"]
r.v[0...2]                    # => ["a", "b", "c"]
```