Gracefully handle unexpected fields in the input.

Mongoid has changed how it handles unknown fields in an `update_attributes` call.  With Mongoid3 it silently persists the extra fields.  With Mongoid4 it raises an error (unless explicitly configured to adopt the Mongoid3 behaviour).  This is a good thing IMO, but it has introduced a regression here. 

As it stands extra fields in the intput are causing a 500 error.  This changes this so that they now return a 422.