'Called just before the HTTP request is sent.
Might alter `request_headers`.
:type request_headers: dict'

def pre_request(self, request_headers):

    request_headers['Accept-Encoding'] = 'identity'
    if self._resume:
        bytes_have = os.path.getsize(self._output_file.name)
        if bytes_have:
            request_headers['Range'] = ('bytes=%d-' % bytes_have)
            self._resumed_from = bytes_have
