Fix rack update support

Currently, you must do:

  curl -X PUT -H 'Accept: application/json' \
              -H 'Content-Type: application/json' \
       -d '{"id: "1", "name": "new-name"}' http://0.0.0.0:6385/v1/racks

but PUT should be on the entity URL itself:

  curl -X PUT -H 'Accept: application/json' \
              -H 'Content-Type: application/json' \
       -d '{"name": "new-name"}' http://0.0.0.0:6385/v1/racks/1