Fix "can't modify frozen object" error

When a `head` request is made the `response[:body]` is `nil` which means
the `result` variable is `nil` which throws a "can't modify frozen
object" error on `result.extend`.

returning a hash rather than `nil` seemed like the best fix with my limited knowledge of how RSolr works but you may be able to come up with a better solution.

To reproduce this try

    RSolr.connect(url: solr_url).head('admin/ping')