Update priority of Content-Type listener

- Should happen between authorization (-600) and validation (-650)
Normalization of mediatype negotiation listeners

- Excecute all listeners except AcceptListener (which is used to
  seed the AcceptableViewModelSelector) during the "route" event, at low
  priority.
- Execute all above listeners at -625, so they execute between HTTP
  method authorization (-600) and input validation (-650).