Enforced presence of a completedBlock in downloadWithURL:options:progress:completed

This method bails out in case of a missing `completedBlock`. While this makes sense (downloading the image and not doing anything with it is pointless), a client passing a `nil` block is not informed about the mistake.

`NSParameterAssert` validates the input and informs the client about bad usage of the API.
@Gabro why do you consider this a misuse? completionBlock means giving back the control to the requester when the async operations is finished. If they decide that nothing needs to happen then, I think it's still a good use.