Adding check for existing destination for COPY, MOVE, PUT and POST w/slug

See https://www.pivotaltracker.com/story/show/70965620 and https://www.pivotaltracker.com/story/show/71632224

* COPY/MOVE to a resource, PUT (w/o properties) and POST (w/slug) now return 409 Conflict if the resource exists.
* Adding ITs to check all of these.