Fix "age" header check in XHR loader

[robwalch]

This PR will...

Use RegEx to match "age" header in XMLHttpRequest.getAllResponseHeaders(), skipping the CDN Tune-In "age" header check in live streams with no "age" in the playlist response header.

Why is this Pull Request needed?

Fixes regression introduced in #3685 in response to #3680 that turns up the Refused to get unsafe header "age"' errors "to 11".

The solution that used getAllResponseHeaders().indexOf('age') matched the word "age" anywhere in the headers as in "max-age" as part of:

cache-control: max-age=2
content-length: 283
content-type: application/vnd.apple.mpegurl
date: Mon, 12 Apr 2021 20:39:28 GMT
expires: Mon, 12 Apr 2021 20:39:30 GMT
last-modified: Mon, 12 Apr 2021 20:39:28 GMT

Resolves issues:

Resolves #3680

Checklist

• changes have been done against master branch, and PR does not conflict