createAction should cope with 201 Created responses that have no body

When creating a new user (against mod-users-bl), the response contains the record, which is then made available as the new 0th element of successfulMutations. However, when creating a new item (against mod-inventory), the response has no body – which is perfectly acceptable behavior in 201 Created response.

Stripes-connect should allow for this eventuality, and in this situation should furnish the client record to the successfulMutations.

(Doing this will allow us to fix UIIT-29, focussing newly created items on creation.)