code: function normalizeContentLengthHeader(headers, body, options = {}) {
    const logger = options.logger || defaultLogger;
  
    const modifiedHeaders = Object.assign({}, headers);
    const calculatedValue = Buffer.byteLength(body);
    const name = caseless(modifiedHeaders).has('Content-Length');
    if (name) {
      const value = parseInt(modifiedHeaders[name], 10);
      if (value !== calculatedValue) {
        modifiedHeaders[name] = `${calculatedValue}`;
        logger.warn(`Specified Content-Length header is ${value}, but the real `
          + `body length is ${calculatedValue}. Using ${calculatedValue} instead.`);
      }
    } else {
      modifiedHeaders['Content-Length'] = `${calculatedValue}`;
    }
    return modifiedHeaders;
  }
docs: Detects an existing Content-Length header and overrides the user-provided
  header value in case it's out of sync with the real length of the body.
  
  @param {Object} headers HTTP request headers
  @param {Buffer} body HTTP request body
  @param {Object} [options]
  @param {Object} [options.logger] Custom logger