CommandHandler avoid endless loop

Background:
I was checking if commands needed to implement a certain interface. Even if it was only an empty one. This should at least force them to be objects. 
https://github.com/qandidate-labs/broadway/blob/master/src/Broadway/CommandHandling/CommandHandlerInterface.php
Turned out that wasn't the case. So I wanted to see what would happen if I send a string as a command.

I was testing this on the command line without a framework.
I expected an exception somewhere but I ended up in an endless loop.

If you check the code the get_class command did return an warning (but no exception), neither did the explode function.
This caused me to endlessly loop the method handle.
This pull request fixed that behaviour.
@sstok CommandNotAnObject so this means that our command bus was able to send a command as a string. But only if our system crashes we would prevent that.

This would mean that every listener of a command should check if it's an object. But commands have rules, they NEED to be objects. The problem is that this code was allowed in the first place.

That's why i believe this system is really fragile . It allows the users to send whatever data they want through the system.