Make sure search request options are passed through.

I think these options are meant to be passed on, not silently ignored. 

I discovered this while trying to perform a multi-index search while retaining the niceities of the Response/Results/Result wrapping by doing something like this:

```ruby
Foo.search({query: ...}, index: [Foo.index_name, Bar.index_name].join(','))
```

(more or less what I used `Tire.search` for before)

However since the options are not being passed on I'm unable to update the `index` field.