4.x - Add check for Slim callable notation if no resolver given

[adriansuter]

PR for #2804

The \Slim\DeferredCallable can be instantiated without callable resolver. If the callable is a string, then in many cases, this should work as expected. For example for function names, or ClassName::MethodName notation. But the Slim callable notation ClassName:MethodName would not work. This PR makes sure that we throw an exception if that is the case.

[coveralls]

Coverage remained the same at 100.0% when pulling 5975762 on adriansuter:patch-deferred-callable into 8ab3432 on slimphp:4.x.

[l0gicgate]

As mentioned in #2804 we should remove DeferredCallable altogether. It's just unnecessary.

[adriansuter]

Closing as we should rather remove the DeferredCallable.