Work around recent Akamai/Microsoft issues

[unshare]

At the moment, Microsoft Office and related casks fail audit due to failure to fetch a product homepage from the main Microsoft site (https://www.microsoft.com/) served by Akamai CDN. The failure is severe and weird: no status code is received as the connection is reset.
The analysis revealed the issue is linked to User-Agent and Accept-Language headers parsing. Homebrew uses involved user agent strings and has language hardcoded as en.
The simplest workaround is to use a simple user agent string when checking homepage availability if the cask is in an audit exceptions list.

Merging this would fix Microsoft Office updates (including Homebrew/homebrew-cask#162671)

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[MikeMcQuaid]

Thanks so much for your first contribution! Without people like you submitting PRs we couldn't run this project. You rock, @unshare!