PDO MySQL fix: Named placeholders with Question mark placeholders throw an Exception

The issue can be reproduced when question mark placeholders and named placeholders are mixed AND question mark placeholder comes after named: 
```php
$Query = (new Query())->from('tbl_user');
$Query->where('id < :id', [':id' => 100]);
$Query->orWhere('id > ?', [1 => 500]);
```

**PDOException: SQLSTATE[HY093]: Invalid parameter number: Columns/Parameters are 1-based**

![pdoexception_-_2014-05-31_15 55](https://cloud.githubusercontent.com/assets/1533818/3139046/a033fc6e-e8c5-11e3-9e92-5de54ae446ca.gif)

Another one example will work as expected (without bug), because question mark goes first: 
```php
$Query = (new Query())->from('tbl_user');
$Query->where('id > ?', [1 => 500]);
$Query->orWhere('id < :id', [':id' => 100]);
```