Use binary_type in MongoDBStore.__getitem__ for Python 2 only (#401)

* Drop binary_type cast in MongoDBStore.__getitem__

* Coerce PyMongo values to `binary_type` on Python 2

On both Python 2 and Python 3, PyMongo converts `binary_type` (i.e.
`bytes`) to BSON type 5 (Binary data) with subtype 0 to store in the
MongoDB instance. When reading that data back out, PyMongo handles BSON
type 5 (Binary data) with subtype 0 differently depending on the Python
version.

On Python 2, it creates a `bson.Binary` instance with the data. Normally
we would coerce this to a `bytes` object using `ensure_bytes`. However
that fails as `bson.Binary` is a subclass of `bytes`. So instead we
explicitly force it to `bytes` (i.e.  `binary_type`). On Python 3,
PyMongo automatically converts the data to `bytes`. Thus we don't need
to do anything there.

ref: http://api.mongodb.com/python/current/python3.html#id3
ref: http://api.mongodb.com/python/current/api/bson/binary.html#bson.binary.Binary

Author: jakirkham

zarr/storage.py

@@ -2147,7 +2147,15 @@ def __getitem__(self, key):
         if doc is None:
             raise KeyError(key)
         else:
-            return binary_type(doc[self._value])
+            value = doc[self._value]
+
+            # Coerce `bson.Binary` to `bytes` type on Python 2.
+            # PyMongo handles this conversion for us on Python 3.
+            # ref: http://api.mongodb.com/python/current/python3.html#id3
+            if PY2:  # pragma: py3 no cover
+                value = binary_type(value)
+
+            return value
 
     def __setitem__(self, key, value):
         value = ensure_bytes(value)