fix(analytics): Fix custom data layer function (#2594)

Change arrow function to a normal function to use the arguments object as described in the gtag documentation. That way, analytics sends data back to Google again.

Fixes #2505

src/analytics/analytics.ts

@@ -65,10 +65,11 @@ export class AngularFireAnalytics {
 
     if (!analyticsInitialized) {
       if (isPlatformBrowser(platformId)) {
-        gtag = (window[GTAG_FUNCTION_NAME] as any) || ((...args: any[]) => {
-          (window[DATA_LAYER_NAME] as any).push(args);
-        });
         window[DATA_LAYER_NAME] = window[DATA_LAYER_NAME] || [];
+        // tslint:disable-next-line: only-arrow-functions
+        gtag = (window[GTAG_FUNCTION_NAME] as any) || (function(..._args: any[]) {
+          (window[DATA_LAYER_NAME] as any).push(arguments);
+        });
         analyticsInitialized = zone.runOutsideAngular(() =>
           new Promise(resolve => {
             window[GTAG_FUNCTION_NAME] = (...args: any[]) => {