Fix sortable behaviour not saving a given rank on an object

Might be related to #443
Personally this is happening to me on a sortable N-N table with extra attributes.

Each time the main object is update, the ArrayCollection is cleared, 
and all N-N rows are inserted by hand with saved values (before clearing the collection)

It ends up having the behaviour ignoring the rank I give to the object.
Merge pull request #621 from tristanbes/patch-1

Fix sortable behaviour not saving a given rank on an object