spotify: more robust `has()` function
spotify: more robust `has()` function

Should prevent the Error from being thrown in #29.

christophior/node-spotify-web,mirego/node-spotify-web,sciencepro/node-spotify-web,Stephen304/yfitops-web,TooTallNate/node-spotify-web,longle255/node-spotify-web,AmpMe/node-spotify-web,airbnb/node-spotify-web,fuzeman/node-spotify-web,pardo/node-spotify-web