When SimplePie parses HTTP headers, it combines multiple values for the same header into a comma-separated string. `WP_SimplePie_File` overrides the parsing, but was leaving them as an array instead.
That lead to a fatal error in PHP 8, because other parts of the codebase ended up passing an array to a function that expected a string.
Props david.binda, litemotiv, inc2734, NicolasKulka, hellofromTonya, mbabker, skithund, SergeyBiryukov, desrosj, timothyblynjacobs.
Fixes#51056. See #51956.
git-svn-id: https://develop.svn.wordpress.org/trunk@49803 602fd350-edb4-49c9-b593-d223f7449a82
