Add missing setter to detectPassiveEvents (close #995)

Author: Paul Boocock

src/js/tracker.js

@@ -1598,6 +1598,7 @@ export function Tracker(functionName, namespace, version, mutSnowplowState, argm
               get: function get() {
                 passive = true;
               },
+              set: function set() {},
             });
             // note: have to set and remove a no-op listener instead of null
             // (which was used previously), becasue Edge v15 throws an error