piston_extended_sticky.json not supported (1.8/1.9)

piston_extended_sticky.json is not supported or not added.

when a sticky piston is extended it must use the textures of a non-sticky piston. It should be able to use separate textures from the normal piston.
The sticky piston can use separate textures when it's not extended already.

[IE: for use with custom texture packs]
i'm looking for distinct (but simple) differences between sticky and normal pistons in all positions/faces. this was the only one i could not accomplish with textures/models. (.png and .json)
If this is already fixed/possible. please advise.
Thank you.

[[started my texture pack using the default 1.9 resource pack downloaded from planetminecraft, and been modifying textures, and a few jsons so i was expecting everything to be there]]