[WIP] Converters

This PR reverts #769 (which tried to fix #768) as this is a breaks BC for the most usual use case of converters (#806).

The decision has been taken (#814) that a converter always run, independently of the available request attribute names or controller argument names.

Because of the above, running the converters as a controller event is not needed anymore, and moving the converters earlier in the process gives more power to the developer (#825 and #826).