'Write an HTTP header block.
:arg start_line: a `.RequestStartLine` or `.ResponseStartLine`.
:arg headers: a `.HTTPHeaders` instance.
:arg chunk: the first (optional) chunk of data.  This is an optimization
so that small responses can be written in the same call as their
headers.
:arg callback: a callback to be run when the write is complete.
The ``version`` field of ``start_line`` is ignored.
Returns a `.Future` if no callback is given.'

def write_headers(self, start_line, headers, chunk=None, callback=None):

    raise NotImplementedError()
