Allow callback to be a string for backwards compatibility & added test c...

I think this will fix the issue in https://github.com/venomous0x/WhatsAPI/issues/559. Basically, I added type hinting to the constructor of the LegacyAdapter, however it was possible (in WhatsAppEvent) to pass in a callback as a string (instead of a callable.) Strings are discouraged as callbacks, however it was supported so this will get it to work again.

Also, I updated the test cases so that this case is tested now.