dojo/store/Memory - assign id when adding objects without ids

[ben hockey]

using a Memory store, i would expect the following to work:

var store = new Memory(),
    ukObject = {name:'UK', type:'country', parent: 1},
    ukID = store.add(ukObject);

// identity is returned as undefined
store.getIdentity(tree.store.get(ukID));

this happens because even though store.add is returning an id, the object being added is not assigned the id.

[Kris Zyp]

In [27072]:

Record generated id in object, fixes #14281 !strict

[Simon Speich]

This also fixes #12835