small bug in OpenDocument.save() with outputfile=io.BytesIO and addsuffix=True

[gamboz]

in this case, the code tries to add a string suffix to the ioBytesIO object and fails odf/opendocument.py lines 575,576:

        if addsuffix:
            outputfile = outputfile + odmimetypes.get(self.mimetype,u'.xxx')

thanks