Allow a path as the return value of the keyForXXX.

Reported as https://github.com/emberjs/data/pull/791

Consider the following JSON: { dashboard: { content: { tabs: [] } } }

App.Dashboard.reopen({
    tabs: DS.hasMany(App.Tab)
});
App.DashboardAdapter.map('App.Dashboard', {
    tabs: { embedded: "always" }
});

App.DashboardSerializer = DS.JSONSerializer.extend({
    keyForHasMany: function(type, name) {
        return 'content.'+name;
    }
});