Fix userExist with PDO

DbUser.php

@@ -250,7 +250,7 @@ public function dropUserQuery($username)
      */
     public function userExist($username)
     {
-        return $this->connection->fetchColumn($this->userExistQuery($username));
+        return $this->connection->query($this->userExistQuery($username))->fetchColumn();
     }
 
     /**

Tests/Units/DbUser.php

@@ -8,6 +8,7 @@
 namespace Shinbuntu\DbUser\Tests\Units;
 
 use atoum;
+use mock\Doctrine\DBAL\Driver\PDOStatement;
 use mock\Doctrine\DBAL\Connection;
 use Shinbuntu\DbUser\DbUser as testedClass;
 
@@ -38,6 +39,9 @@ private function getConnection()
         $this->connection->getMockController()->quote = function ($input) {
             return '"'.addslashes($input).'"';
         };
+        $this->connection->getMockController()->query = function () {
+            return new PDOStatement();
+        };
 
         $this->mockGenerator->unshuntParentClassCalls();