Currently, only `Livecheck::Strategy::PAGE_HEADERS_CURL_ARGS` uses
the `--silent` option and `PAGE_CONTENT_CURL_ARGS` does not (though
there's no intention behind this omission). However, the
`#page_content` method should also use the `--silent` flag, to
prevent progress bar text (`#=#=#`, etc.) from appearing in output.
This is an issue because the regex that's used to identify `curl`
error messages in `stderr` (`^curl:.+$/`) will fail if leading
progress bar text is present. This leads to an ambiguous "cURL
failed without a detectable error" message instead of the actual
error message(s) from `curl`.
This commit addresses the issue by adding `--silent` to
`Livecheck::Strategy::DEFAULT_CURL_ARGS`, which both
`PAGE_HEADERS_CURL_ARGS` and `PAGE_CONTENT_CURL_ARGS` inherit.
cb537eae52