In the [AWS.Request.send() docs](https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/Request.html#send-property), we can see that the callback is optional but this lib incorrectly assumes there will always be a callback [here](https://github.com/dwyl/aws-sdk-mock/blob/e3377a5ebdec678db535e426c6cc37ba55542598/index.js#L227).