Fix "age" header check in XHR loader

Fixes regression introduced in #3685 in response to #3680
Resolves #3680

src/utils/xhr-loader.ts

@@ -8,6 +8,8 @@ import type {
 } from '../types/loader';
 import { LoadStats } from '../loader/load-stats';
 
+const AGE_HEADER_LINE_REGEX = /^age:\s*[\d.]+\s*$/m;
+
 class XhrLoader implements Loader<LoaderContext> {
   private xhrSetup: Function | null;
   private requestTimeout?: number;
@@ -253,7 +255,7 @@ class XhrLoader implements Loader<LoaderContext> {
     let result: number | null = null;
     if (
       this.loader &&
-      this.loader.getAllResponseHeaders().indexOf('age') >= 0
+      AGE_HEADER_LINE_REGEX.test(this.loader.getAllResponseHeaders())
     ) {
       const ageHeader = this.loader.getResponseHeader('age');
       result = ageHeader ? parseFloat(ageHeader) : null;