Form\Collection: allow create new objects

The commit https://github.com/zendframework/zf2/commit/b0c03a331565960599de338311cc56cb940c4655 , merged in 2.1.4, introduced a compatibility break: before that commit, the behaviour of collection was totally different. If you expecially used collections with Doctrine Entities, this is a huge change.

This PR reintroduce the ability to create new objects if requested, and leave the new behaviour by default.