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 when 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). 
```php
$Query = (new Query())->from('tbl_user');
$Query->where('id > ?', [1 => 500]);
$Query->orWhere('id < :id', [':id' => 100]);
```