Always prepare Connection params from Client options

Copying my comment from FriendsOfSymfony/FOSElasticaBundle#305, which prompted this PR:

> Looking at the constructor for [Elastica\Client](https://github.com/ruflin/Elastica/blob/master/lib/Elastica/Client.php) and [Elastica\Connection](https://github.com/ruflin/Elastica/blob/master/lib/Elastica/Connection.php), I think this is an issue with Elastica. This bundle always passes an array of `$servers` to the Client constructor. When that defers to the `Connection::create()` factory method (and the constructor in turn), the `curl`, `headers`, and `url` options are not given special consideration as they are in `Client::_configureParams()`. I'll open up a PR on Elastica and try to correct this, as it looks like an oversight.

I ended up renaming a protected method, which may be considered a BC break.

An alternative solution might have been to do this extraction in the Connection class, but it's likely only relevant to how Clients are constructed.