'Implements the PEP-3333 `start_response` protocol.'

def __call__(self, status, headers, exc_info=None):

    self._called += 1
    self.status = status
    self.headers = [(name.lower(), value) for (name, value) in headers]
    self.headers_dict = util.CaseInsensitiveDict(headers)
    self.exc_info = exc_info
