fixed PLAY-497

Using the same .jpg image crushes Player
@mcm69 I think there should be no `!` (_not_) operator in front, then — since things are a bit complex here. 

The important thing is that we get in this branch of code actually when someone's already requested same resource, but still haven't received it. If there's no-one waiting for this recourse, then it's either in cache or in errors, or it wasn't ever requested, but all these cases are covered in the code above.

The question is, should we subscribe to the same resource if the same `loadOrGet` was called again for the same player instance (`subject_id`) while the previous request wasn't finished (object is neither in cache or in errors). Since it was called second time, probably user expects it to be called twice (it has handlers inside which i.e. create one more Image Element or Audio Element, so it's ok). So yeah, we need to subscribe to call this handlers just after the first ever loader got the resource (or error).

And if some different `loadOrGet()` was called for the same URL, seems we also need to subscribe to this request to execute this code when it will be received.

So it was a cause of the bug, seems, yes — but then, I suppose, it should always _go_ inside and subscribe, if someone's waiting for same resource, while `!` prevents it from doing so.