Minor internal change.

Check directly the private parameters array instead of passing through
method dispatching for the outer __isset() method.

lib/Predis/Connection/ConnectionParameters.php

@@ -153,7 +153,7 @@ class ConnectionParameters implements ConnectionParametersInterface
      */
     public function __get($parameter)
     {
-        if (isset($this->{$parameter})) {
+        if (isset($this->parameters[$parameter])) {
             return $this->parameters[$parameter];
         }
     }