`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.
This addresses the issue by updating `curl_headers` to handle `POST`
requests and skip the `GET` retry logic.
9096a111d7