fetch plugin test: testing what happens on 400+ status codes (and rising coverage to 100%; ref. #8)

__tests__/plugins/fetch.test.js

@@ -62,4 +62,26 @@ describe("plugin: fetch", () => {
     expect(response.headers.has('X-LibResilient-Etag')).toEqual(true)
     expect(response.headers.get('X-LibResilient-ETag')).toEqual('TestingETagHeader')
   });
+  
+  test("it should throw an error when HTTP status is >= 400", async () => {
+    
+    global.fetch.mockImplementation((url, init) => {
+            const response = new Response(
+                                new Blob(
+                                  ["Not Found"],
+                                  {type: "text/plain"}
+                                ),
+                                {
+                                  status: 404,
+                                  statusText: "Not Found",
+                                  url: url
+                                });
+            return Promise.resolve(response);
+          });
+    
+    require("../../plugins/fetch.js");
+
+    expect.assertions(1)
+    expect(self.LibResilientPlugins[0].fetch('https://resilient.is/test.json')).rejects.toThrow(Error)
+  });
 });