Allow properly bound attributes for HTMLBars link-to helper.

Initially I used `readUnwrappedModel`, but that replaces the stream with a static string (clearly will not be bound at that point). This uses similar logic as the Handlebars link-to (and `readUnwrappedModel`) to find the lowest unwrapped model, but preserves things as a stream.

I came accross this while working through the `{{link-to}}` tests in the `ember` package (but they cannot yet be added due to other issues).