utils/curl: use specs when checking http content problems

[Rylan12]

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See Homebrew/discussions#576

Currently, specs passed to the url method are passed to curl when fetching the resource but aren't passed to curl when auditing the resource's url. For example, the user who opened Homebrew/discussions#576 needed to use the --referrer flag in curl. That meant that brew audit --online was failing even though the url was reachable when actually running brew install.

This PR passes those specs on to Utils::Curl.curl_check_http_content to ensure that the check will pass in situations like these.

[BrewTestBot]

Review period will end on 2021-01-26 at 00:00:00 UTC.

[BrewTestBot]

Review period skipped due to critical label.

[MikeMcQuaid]

Thanks again @Rylan12!

[cesarcoatl]

Great work, @Rylan12! 🥇