diff -r 34716fd837a4 -r be944660c56a wp/wp-includes/class-wp-simplepie-file.php
--- a/wp/wp-includes/class-wp-simplepie-file.php	Tue Dec 15 15:52:01 2020 +0100
+++ b/wp/wp-includes/class-wp-simplepie-file.php	Wed Sep 21 18:19:35 2022 +0200
@@ -10,6 +10,9 @@
 /**
  * Core class for fetching remote files and reading local files with SimplePie.
  *
+ * This uses Core's HTTP API to make requests, which gives plugins the ability
+ * to hook into the process.
+ *
  * @since 2.8.0
  *
  * @see SimplePie_File
@@ -21,15 +24,17 @@
 	 *
 	 * @since 2.8.0
 	 * @since 3.2.0 Updated to use a PHP5 constructor.
+	 * @since 5.6.1 Multiple headers are concatenated into a comma-separated string,
+	 *              rather than remaining an array.
 	 *
 	 * @param string       $url             Remote file URL.
-	 * @param integer      $timeout         Optional. How long the connection should stay open in seconds.
+	 * @param int          $timeout         Optional. How long the connection should stay open in seconds.
 	 *                                      Default 10.
-	 * @param integer      $redirects       Optional. The number of allowed redirects. Default 5.
+	 * @param int          $redirects       Optional. The number of allowed redirects. Default 5.
 	 * @param string|array $headers         Optional. Array or string of headers to send with the request.
 	 *                                      Default null.
 	 * @param string       $useragent       Optional. User-agent value sent. Default null.
-	 * @param boolean      $force_fsockopen Optional. Whether to force opening internet or unix domain socket
+	 * @param bool         $force_fsockopen Optional. Whether to force opening internet or unix domain socket
 	 *                                      connection or not. Default false.
 	 */
 	public function __construct( $url, $timeout = 10, $redirects = 5, $headers = null, $useragent = null, $force_fsockopen = false ) {
@@ -60,8 +65,32 @@
 			if ( is_wp_error( $res ) ) {
 				$this->error   = 'WP HTTP Error: ' . $res->get_error_message();
 				$this->success = false;
+
 			} else {
-				$this->headers     = wp_remote_retrieve_headers( $res );
+				$this->headers = wp_remote_retrieve_headers( $res );
+
+				/*
+				 * SimplePie expects multiple headers to be stored as a comma-separated string,
+				 * but `wp_remote_retrieve_headers()` returns them as an array, so they need
+				 * to be converted.
+				 *
+				 * The only exception to that is the `content-type` header, which should ignore
+				 * any previous values and only use the last one.
+				 *
+				 * @see SimplePie_HTTP_Parser::new_line().
+				 */
+				foreach ( $this->headers as $name => $value ) {
+					if ( ! is_array( $value ) ) {
+						continue;
+					}
+
+					if ( 'content-type' === $name ) {
+						$this->headers[ $name ] = array_pop( $value );
+					} else {
+						$this->headers[ $name ] = implode( ', ', $value );
+					}
+				}
+
 				$this->body        = wp_remote_retrieve_body( $res );
 				$this->status_code = wp_remote_retrieve_response_code( $res );
 			}