use purgeConnection instead of destroy on restoreUser error

Author: seangarner

lib/Pool.js

@@ -111,7 +111,7 @@ Pool.prototype.releaseConnection = function releaseConnection(connection) {
     if (connection._purge) {
       // restore the switched user
       this.restoreUser(connection, function (err) {
-        if (err) return connection.destroy();
+        if (err) return this._purgeConnection(connection);
         connection._pool.releaseConnection(connection);
       });
       return;