fixed karma spec

spec/javascripts/projects_dropdown/service/projects_service_spec.js

@@ -12,12 +12,21 @@ FREQUENT_PROJECTS.MAX_COUNT = 3;
 
 describe('ProjectsService', () => {
   let service;
+  let originalGon;
 
   beforeEach(() => {
-    gon.api_version = currentSession.apiVersion;
+    originalGon = window.gon;
+
+    window.gon = Object.assign({}, {
+      api_version: currentSession.apiVersion,
+    });
     service = new ProjectsService(currentSession.username);
   });
 
+  afterEach(() => {
+    window.gon = originalGon;
+  });
+
   describe('contructor', () => {
     it('should initialize default properties of class', () => {
       expect(service.isLocalStorageAvailable).toBeTruthy();