msgpack: fix datetime serialization
msgpack: fix datetime serialization

If the datetime is serialized with python2 and deserializer
with python3 we got KeyError, because python3 will read
the data written with python2 as bytes and doesn't convert it in unicode.

So ensure we use always unicode everywhere.

Change-Id: I4bee7369a278fa7705cde860c68e133cee2fa79e
Closes-bug: #1556081

openstack/oslo.serialization