PayloadOf.java:41: Add tests for PayloadOf.

The puzzle `108-8ad4a268` from #108 has to be resolved:

https://github.com/amihaiemil/docker-java-api/blob/ab0d78ec783de53306d63a66dd459598d58dc8c4/src/main/java/com/amihaiemil/docker/PayloadOf.java#L41-L41

The puzzle was created by George Aristy on 31-May-18.

If you have any technical questions, don't ask me, submit new tickets instead. The task will be \"done\" when the problem is fixed and the text of the puzzle is _removed_ from the source code. Here is more about [PDD](http://www.yegor256.com/2009/03/04/pdd.html) and [about me](http://www.yegor256.com/2017/04/05/pdd-in-action.html).