Set target of ArrayController generated by #each helper to current controller

This ensures action dispatch works as expected when specifying `itemController` via the `{{#each}}` helper.

In the current implementation, specifying `itemController` within the definition of an `ArrayController` works very differently than specifying it via a parameter to `{{#each}}`. In the former case, `{{action}}`s bubble up to the parent controller, route, etc. but in the latter stop at the `itemController`.

Fixes #1989