'Called when the HTTP headers have been received and parsed.
:arg start_line: a `.RequestStartLine` or `.ResponseStartLine`
depending on whether this is a client or server message.
:arg headers: a `.HTTPHeaders` instance.
Some `.HTTPConnection` methods can only be called during
``headers_received``.
May return a `.Future`; if it does the body will not be read
until it is done.'

def headers_received(self, start_line, headers):

    pass
