Assert MsgPack round-trips bytes objects correctly

Previously MsgPack was turning bytes objects to unicode objects when
round-tripping them. However this has been fixed in the latest version
of Numcodecs. So correct this test now that MsgPack is working
correctly.

Author: jakirkham

zarr/tests/test_core.py

@@ -982,7 +982,7 @@ def test_object_arrays(self):
         z[0] = 'foo'
         assert z[0] == 'foo'
         z[1] = b'bar'
-        assert z[1] == 'bar'  # msgpack gets this wrong
+        assert z[1] == b'bar'
         z[2] = 1
         assert z[2] == 1
         z[3] = [2, 4, 6, 'baz']