code: function processBody(opts, body) {
    if (body === undefined) {
      return '';
    }
    const handlers = Object.assign({}, serializers, opts.serializers);
    const headers = typeof opts.headers === 'object' ? opts.headers : {};
    const contentType = Object.keys(headers).reduce((type, header) => {
      if (typeof type === 'string') {
        return type;
      }
      if (
        header.match(/^content-type$/i) &&
        typeof headers[header] === 'string'
      ) {
        return headers[header].toLowerCase();
      }
      return type;
    }, undefined);
  
    const handler = handlers[contentType];
  
    if (typeof handler === 'function') {
      return String(handler(body));
    }
    if (typeof body === 'object') {
      return JSON.stringify(body);
    }
    return String(body);
  }
docs: Converts the body value to a string using optional serializers
  @param {*} opts
  @param {*} body