WSGI callable to begin the HTTP response.

def start_response(self, status, headers, exc_info = None):
    """WSGI callable to begin the HTTP response."""
    if self.started_response:
      if not exc_info:
        raise AssertionError("WSGI start_response called a second "
                   "time with no exc_info.")
      else:
        try:
          raise exc_info[0], exc_info[1], exc_info[2]
        finally:
          exc_info = None
    self.started_response = True
    self.status = status
    self.outheaders.extend(headers)

    return self.write_buffer.write