<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def add_header(self, key, value):
 """ Add a response header """


        assert isinstance(key, str), 'header key must be of type str'
        assert isinstance(value, str), 'header value must be of type str'
        self.headers[key] = value