'Modify the request headers'

def request(self, method, request_uri, headers, content):

    keys = _get_end2end_headers(headers)
    keylist = ''.join([('%s    ' % k) for k in keys])
    headers_val = ''.join([headers[k] for k in keys])
    created = time.strftime('%Y-%m-%dT%H:%M:%SZ', time.gmtime())
    cnonce = _cnonce()
    request_digest = ('%s:%s:%s:%s:%s' % (method, request_uri, cnonce, self.challenge['snonce'], headers_val))
    request_digest = hmac.new(self.key, request_digest, self.hashmod).hexdigest().lower()
    headers['Authorization'] = ('HMACDigest    username="%s",    realm="%s",    snonce="%s",    cnonce="%s",    uri="%s",    created="%s",    response="%s",    headers="%s"' % (self.credentials[0], self.challenge['realm'], self.challenge['snonce'], cnonce, request_uri, created, request_digest, keylist))
