Some additional "safe words" for PlatformInterface#quoteIdentifierInFragment(...) call in Zend\Db\Sql\Select#processJoins(...)

The [list of the safe words](https://github.com/zendframework/zf2/blob/master/library/Zend/Db/Sql/Select.php#L751) passed to (the implementations of) the Zend\Db\Adapter\Platform\PlatformInterface#quoteIdentifierInFragment(...) extended, in order to prevent [issues](http://stackoverflow.com/q/16060679/2019043) with quoting of IS, NULL, single quote(-s), double quote(-s), and != compare operator.