fix handler compiler pass

I was getting this error:

```
[{"message":"Warning: call_user_func() expects parameter 1 to be a valid callback, class 'pc_product_image_serializer' not found in \/home\/bart\/workspace\/macedes.com\/vendor\/jms\/serializer-bundle\/JMS\/SerializerBundle\/Serializer\/GraphNavigator.php line 159","class":"ErrorException","trace":....]
```

I was able fix it using the change in this PR, yet i find it strange that noone else ever encountered this... Is this fix legit, or am I actually taking a wrong approach here?