'Return first matching header value for \'name\', or \'value\'
If there is no header named \'name\', add a new header with name \'name\'
and value \'value\'.'

def setdefault(self, name, value):

    result = self.get(name)
    if (result is None):
        self._headers.append((name, value))
        return value
    else:
        return result
