Zend Db Query Builder Optimisation

Found that the query builder was spending a long time building queries with large arrays in the where clause. After a little bit of profiling noticed that line 111 was been called a lot. `array_search` is an expensive lookup, especially as the return value wasn't been used beyond checking it wasn't false, so replaced with a less expensive `isset`.
`null` values :-)

```php
$data = ['foo' => null];
var_dump(isset($data['foo']));
```
http://3v4l.org/ZdNWD