It is implemented since two months or so 
Just remember that openHAB has no “stable” / “unstable” branches that gets any backports of later changes (we just call it “stable”). You must be on openHAB snapshots to get those changes.
The timing is just really bad at the moment, because openHAB is in the process of changing the build system. The snapshots sporadically do not work since that process started end of January and is finished by maybe end of the month.
Cheers, David