better implementation of the fetch test

__tests__/plugins/fetch.test.js

@@ -1,15 +1,21 @@
 const makeServiceWorkerEnv = require('service-worker-mock');
 
 global.fetch = require('node-fetch');
-jest.mock('node-fetch', () => {
-  const context = {
-    then: jest.fn().mockImplementation(() => {
-      const response = { test: "success" };
-      return Promise.resolve(response);
-    })
-  };
-  return jest.fn(() => context);
-});
+jest.mock('node-fetch')
+
+global.fetch.mockImplementation((url, init) => {
+          const response = new Response(
+                            new Blob(
+                              [JSON.stringify({ test: "success" })],
+                              {type: "application/json"}
+                            ),
+                            {
+                              status: 200,
+                              statusText: "OK",
+                              headers: []
+                            });
+          return Promise.resolve(response);
+        });
 
 describe("plugin: fetch", () => {
   beforeEach(() => {
@@ -33,8 +39,8 @@ describe("plugin: fetch", () => {
   test("it should return data from fetch()", async () => {
     require("../../plugins/fetch.js");
 
-    const returnedJSON = await self.LibResilientPlugins[0].fetch('https://resilient.is/test.json');
+    const response = await self.LibResilientPlugins[0].fetch('https://resilient.is/test.json');
     expect(fetch).toHaveBeenCalled();
-    expect(returnedJSON).toEqual({test: "success"})
+    expect(await response.json()).toEqual({test: "success"})
   });
 });