[dojo.data] JsonItemStore only works with attribute-name-strings, not attribute objects

[skinner]

In the dojo.data.core.Read API, many of the methods take an argument that either can be an attribute-name-string (like "population") or can be an item that represent that attribute (the population item). The JsonItemStore? methods work correctly if you pass in attribute-name-strings, but they fail if you try to pass in attribute objects.

[peller]

See #2704