'Create message string from directives.
@param directives: dictionary of directives (names to their values).
For certain directives, extra quotes are added, as
needed.
@type directives: C{dict} of C{str} to C{str}
@return: message string.
@rtype: C{str}.'

def _unparse(self, directives):

    directive_list = []
    for (name, value) in iteritems(directives):
        if (name in ('username', 'realm', 'cnonce', 'nonce', 'digest-uri', 'authzid', 'cipher')):
            directive = ((name + '=') + value)
        else:
            directive = ((name + '=') + value)
        directive_list.append(directive)
    return ','.join(directive_list)
