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.

Keep in mind though i'm testing this in pure php with the command line.
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.
if possible, throw a specialized exception here. `\InvalidArgumentException` is not specific enough