Description:
------------
A pdo::exec call fails when i try to execute sequential query ( 'OPTIMIZE TABLE' queries )

Reproduce code:
---------------
$dsn = 'mysql:dbname=my_database;host=localhost';
$user = 'root';
$password = '';

try {
   $dbh = new PDO($dsn, $user, $password);
} catch (PDOException $e) {
   echo 'Connection failed: ' . $e->getMessage();
}

$dbh->exec( 'OPTIMIZE TABLE my_table' );
print_r($dbh->errorInfo());

$dbh->exec( 'OPTIMIZE TABLE my_table_1' );
print_r($dbh->errorInfo());

Expected result:
----------------
I expected that the second call to $dbh->exec will be successful as the first one, but the second fails.

Actual result:
--------------
Printing the array returned from $dbh->errorInfo() i obtain for the first call to pdo::exec 

Array
(
    [0] => 00000
)

but for the second call


Array
(
    [0] => HY000
    [1] => 2014
    [2] => Cannot execute queries while other unbuffered queries are active.  Consider using PDOStatement::fetchAll().  Alternatively, if your code is only ever going to run against mysql, you may enable query buffering by setting the PDO_MYSQL_ATTR_USE_BUFFERED_QUERY attribute.
)