This bug still exists in Python 2.7.10 with optparse version 1.5.3.
When the default_value is not ASCII encoded, it would raise `UnicodeEncodeError: 'ascii' codec can't encode characters`

this error is due to the `str` usage in `expand_default` method:

    def expand_default(self, option):
        if self.parser is None or not self.default_tag:
            return option.help

        default_value = self.parser.defaults.get(option.dest)
        if default_value is NO_DEFAULT or default_value is None:
            default_value = self.NO_DEFAULT_VALUE

        return option.help.replace(self.default_tag, str(default_value))