oops, stupid me, this a 3.0 issue..., well seems the str() conversion is
done as well on the 3.0 io module:

"""
class StringIO(TextIOWrapper):
   def __init__(self, initial_value="", encoding="utf-8",
                 errors="strict", newline="\n"):
        super(StringIO, self).__init__(BytesIO(),
                                       encoding=encoding,
                                       errors=errors,
                                       newline=newline)
        if initial_value:
            if not isinstance(initial_value, str):
                initial_value = str(initial_value)
"""