Merge slaves discovered options with original

This is intended to fix following case:

```ruby
redis = Redis.new(url: "redis://localhost:6379/2", master_name: "master", sentinels: ["sentinel://localhost:26379", "sentinel://localhost:36379"])
```

As you can see, I also specified connection url which contains database number, 2.
Before this patch:

```ruby
> redis.all_clients
=> [#<Redis client v3.1.0 for redis://127.0.0.1:6379/2>, #<Redis client v3.1.0 for redis://127.0.0.1:6380/0>]
```

After patch:

```ruby
> redis.all_clients
=> [#<Redis client v3.1.0 for redis://127.0.0.1:6379/2>, #<Redis client v3.1.0 for redis://127.0.0.1:6380/2>]
```

However, I'm not sure if this is correct way to do things, and how should I properly test it.
What do you think?