fixed missing validation of available zoomKey option values

lib/timeline/optionsTimeline.js

@@ -113,7 +113,7 @@ let allOptions = {
   type: {string},
   width: {string, number},
   zoomable: {boolean},
-  zoomKey: {string},
+  zoomKey: {string: ['ctrlKey', 'altKey', 'metaKey', '']},
   zoomMax: {number},
   zoomMin: {number},