_assertBody(body, res) {
    const isRegexp = body instanceof RegExp;

    // parsed
    if (typeof body === 'object' && !isRegexp) {
      try {
        deepStrictEqual(body, res.body);
      } catch (err) {
        const a = inspect(body);
        const b = inspect(res.body);
        return error('expected ' + a + ' response body, got ' + b, body, res.body);
      }
    } else if (body !== res.text) {
      // string
      const a = inspect(body);
      const b = inspect(res.text);

      // regexp
      if (isRegexp) {
        if (!body.test(res.text)) {
          return error('expected body ' + b + ' to match ' + body, body, res.body);
        }
      } else {
        return error('expected ' + a + ' response body, got ' + b, body, res.body);
      }
    }
  }
Perform assertions on a response body and return an Error upon failure.

@param {Mixed} body
@param {Response} res
@return {?Error}
@api private