py3: fix salt/payload.py
py3: fix salt/payload.py

msgpack encodes strings using utf-8 but, unless instructed otherwise,
unpacks said encoded strings as raw bytes. In python 2 this does not
matter since str is bytes. In python 3 it does matter, and decoding
strings as utf-8 works for both pythons.

saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt,saltstack/salt