Comment 16 for bug 1397142

>Global application shortcuts still work correctly with bluetooth media buttons. So for me I worked >around this problem particularly with rhythmbox by adding global application shortcut to command >"rhythmbox-client --play-pause".

This is a good suggestion. A general workaround for all programs (at least in 16.04) is to, in the "Keyboards" utility, under the "Shortcuts" tab, in the "Sound and Media" section, change "Play (or play/pause)" to the the Bluetooth event. On mine, this changed it from "Audio play" to "Audio pause". Now, in Spotify, my bluetooth headphones can both pause and play the music (although I have to hit it twice for each).