Description:
------------
When \PDO::ATTR_EMULATE_PREPARES == false, and the initial connection is created with "some_db" context, the PDO is unable to run the query for changing the database (use another_db) and returns an error:

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.
)


Test script:
---------------
function test_pdo($prepares)
{
	echo 'Testing PDO with \PDO::ATTR_EMULATE_PREPARES=' . ($prepares ? 'true' : 'false') . "\n\n";
	$options[\PDO::ATTR_EMULATE_PREPARES] = $prepares;
	$db = new PDO('mysql:host=some.host.net;dbname=some_db;charset=utf8','user','password',$options);
	$result = $db->query('use ms_system');
	echo 'After PDO::query(use another_db): ';
	echo $result === false ? 'PDO ERROR' : 'PDO is OK';
	echo "\n";
	$stmt = $db->prepare('use ms_system');
	$result = $stmt->execute();
	echo 'After PDO::prepare(use another_db); PDO::execute(): ';
	if ($result === false)	{
		echo 'PDO ERROR - ' . print_r($stmt->errorInfo(), true);
	} else	{
		echo 'PDO is OK';
	}
	echo "\n---------------------------------------------------\n\n";
}
echo '<xmp>'; test_pdo(true); test_pdo(false);

Expected result:
----------------
<xmp>Testing PDO with \PDO::ATTR_EMULATE_PREPARES=true

After PDO::query(use another_db): PDO is OK
After PDO::prepare(use another_db); PDO::execute(): PDO is OK
---------------------------------------------------

Testing PDO with \PDO::ATTR_EMULATE_PREPARES=false

After PDO::query(use another_db): PDO is OK
After PDO::prepare(use another_db); PDO::execute(): PDO is OK

---------------------------------------------------