Non-rdf resources emit /fcr:metadata events instead of /jcr:content

Fixes https://www.pivotaltracker.com/story/show/81279166
On binary update events, you appear to be sending a message with the binary path.
In the case where the event is on the description (i.e. /fcr:metadata) the message is with the description path.
This does not seem to be in line with your previous comments in this thread:
https://github.com/fcrepo4/fcrepo4/pull/582#discussion_r19291816