jsonrpc: fix Player.GetItem for PVR channels not providing extended details like plot, genre etc.

This fixes a regression introduced by #5522 which only considered `PVR.GetChannels` but not the impact on `Player.GetItem`. Right now it's not possible to get extended information like plot, genre etc of the currently playing PVR channel anymore through the basic properties (which worked fine before).

This is not a nice fix since the current EPG object is serialized twice:
* once into the main result object to get direct access to plot, genre etc
* once into the `broadcastnow` where only the PVR channel default of the current EPG object are serialized and belongs together with `broadcastnext` which provides the EPG details of the next broadcast on the same channel.

But it's the easiest fix I could come up with that contains the same code as the old and the new behaviour.