curl_headers: Handle POST requests

[samford]

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Livecheck::Strategy.page_headers uses Utils::Curl.curl_headers but the method only handles HEAD and GET requests. I recently added POST support to livecheck but forgot to update curl_headers in the process, so livecheck blocks using the HeaderMatch strategy along with post_form or post_json will fail because curl doesn't allow both --head and --data/--json arguments (e.g., Homebrew/homebrew-cask#204216).

This addresses the issue by updating curl_headers to handle POST requests and skip the GET retry logic.

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The aforementioned effect-house PR also demonstrated that some servers will return an unexpected response if a Content-Length header isn't included in the POST request, so this adds the header to the post_args array when post_form or post_json are used.

[MikeMcQuaid]

Looks good, thanks @samford!