Add a test for the 404 check

Check was added in #4, this adds a test for it.

Also made it consistently return null, both when the downloaded file is not an icon and when a 404 is received.

Previously it was returning undefined for the latter.

src/modules/download/downloadIcon.js

@@ -30,7 +30,7 @@ function downloadIcon(iconUrl) {
 
     return iconData.then(iconData => {
         if (!iconData) {
-            return;
+            return null;
         }
 
         const fileDetails = fileType(iconData);

test/downloadIcons.js

@@ -7,6 +7,7 @@ const expect = chai.expect;
 
 const ICON_URL = 'https://web.whatsapp.com/favicon.ico';
 const NOT_ICON_URL = 'http://jiahaog.github.io/nativefier-icons';
+const STATUS_NOT_FOUND_URL = 'http://jiahaog.github.io/nativefier-icons/pleasegivemea404';
 
 const ICON_PATH = path.join(__dirname, '..','out', 'test_icon.ico');
 
@@ -31,5 +32,14 @@ describe('Download Icons', function() {
                 done();
             })
             .catch(done);
-    })
+    });
+
+    it('Will return null if the icon ', function(done) {
+        downloadIcon(STATUS_NOT_FOUND_URL)
+            .then(icon => {
+                expect(icon).to.equal(null, 'Invalid icon should return null');
+                done();
+            })
+            .catch(done);
+    });
 });