Return the list of headers as two-tuples

def build_headers(self):
        '''
        Return the list of headers as two-tuples
        '''
        if not 'Content-Type' in self.headers:
            content_type = self.content_type
            if self.encoding != DEFAULT_ENCODING:
                content_type += '; charset=%s' % self.encoding
            self.headers['Content-Type'] = content_type

        headers = list(self.headers.items())
        # Append cookies
        headers += [
            ('Set-Cookie', cookie.OutputString())
            for cookie in self.cookies.values()
        ]
        return headers