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`.

Author: jakirkham

zarr/meta.py

@@ -5,24 +5,20 @@
 
 
 import numpy as np
+from numcodecs.compat import ensure_bytes
 
 
-from zarr.compat import PY2, binary_type, Mapping
+from zarr.compat import PY2, Mapping
 from zarr.errors import MetadataError
 
 
 ZARR_FORMAT = 2
 
 
 def ensure_str(s):
-    if PY2:  # pragma: py3 no cover
-        # noinspection PyUnresolvedReferences
-        if isinstance(s, buffer):  # noqa
-            s = str(s)
-    else:  # pragma: py2 no cover
-        if isinstance(s, memoryview):
-            s = s.tobytes()
-        if isinstance(s, binary_type):
+    if not isinstance(s, str):
+        s = ensure_bytes(s)
+        if not PY2:  # pragma: py2 no cover
             s = s.decode('ascii')
     return s