Dismax RequestBuilder used even if edismax is specified

Hi,

A bugfix for Dismax was used even if edismax was specified

To reproduce : 
$client = new Solarium\Client($config);
$query = $client->createSelect();
$edismax = $query->getEDisMax();
$edismax->setBoostFunctionsMult("sqrt(salecount)");

Then check query, boost is absent from query because Dismax RequestBuilder is used rather than Edismax RequestBuilder