Fix #1951: Allow setting ATTR_STATEMENT_CLASS back to PDOStatement

Currently HHVM strictly enforces that the statement class be a descendent of PDOStatement. While the PHP Documentation states that it has to be a _subclass_, I believe this is subclass in the sense implied by Liskov's OO substitution principle. Thus, the PDOStatement class should be accepted.

Zend does accept the PDOStatement class without warning or notice.