Simplify `ensure_str` in `zarr.meta`
Simplify `ensure_str` in `zarr.meta`

If the data is already a `str` instance, turn `ensure_str` into a no-op.
For all other cases, make use of Numcodecs' `ensure_bytes` to aid
`ensure_str` in coercing data through the buffer protocol. If we are on
Python 3, then decode the `bytes` object to a `str`.

zarr-developers/zarr-python,alimanfoo/zarr