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.
I have to agree with @rivera-ernesto. Usually, it's a better practice to check for completion blocks and allow people not passing one. Gives more flexibility. Better than enforcing someone to pass in an empty non-nil block.