Fix Take's "back"

Trivial fix. Reproducing it required a non-sliceable (or infinite) RA range. `Take` *should* be bidirectinal, but because the "write" version of `back` was not marked as `@property`, calling it produced a `Error: cannot overload both property and non-property functions`. This meant the resulting range was not bidirectional.

This fixes that. Also added a unittest.