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).
Yeah, the helper I was removing does effectively the same thing, but it actually reads the stream.  I am just making a "deeper" stream here.  I've definitely got a list of areas to improve after the initial integration efforts.