Ensure prefer_form_input_filter is set to true

From zf 2.2.4 IIRC, prefer_form_input_filter is set to true by default.

https://github.com/zendframework/zf2/pull/5479 fixed quite some things, and introduced untested breaks when generating forms from annotations with zf 2.3

This PR makes sure the generated form has prefer_form_input_filter to true.