diff -r 490d5cc509ed -r cf61fcea0001 wp/wp-includes/class-wp-http-ixr-client.php
--- a/wp/wp-includes/class-wp-http-ixr-client.php	Tue Jun 09 11:14:17 2015 +0000
+++ b/wp/wp-includes/class-wp-http-ixr-client.php	Mon Oct 14 17:39:30 2019 +0200
@@ -46,6 +46,9 @@
 		$this->timeout = $timeout;
 	}
 
+	/**
+	 * @return bool
+	 */
 	public function query() {
 		$args = func_get_args();
 		$method = array_shift($args);
@@ -61,15 +64,27 @@
 		);
 
 		// Merge Custom headers ala #8145
-		foreach ( $this->headers as $header => $value )
+		foreach ( $this->headers as $header => $value ) {
 			$args['headers'][$header] = $value;
+		}
 
-		if ( $this->timeout !== false )
+		/**
+		 * Filters the headers collection to be sent to the XML-RPC server.
+		 *
+		 * @since 4.4.0
+		 *
+		 * @param array $headers Array of headers to be sent.
+		 */
+		$args['headers'] = apply_filters( 'wp_http_ixr_client_headers', $args['headers'] );
+
+		if ( $this->timeout !== false ) {
 			$args['timeout'] = $this->timeout;
+		}
 
 		// Now send the request
-		if ( $this->debug )
+		if ( $this->debug ) {
 			echo '<pre class="ixr_request">' . htmlspecialchars($xml) . "\n</pre>\n\n";
+		}
 
 		$response = wp_remote_post($url, $args);
 
@@ -85,8 +100,9 @@
 			return false;
 		}
 
-		if ( $this->debug )
+		if ( $this->debug ) {
 			echo '<pre class="ixr_response">' . htmlspecialchars( wp_remote_retrieve_body( $response ) ) . "\n</pre>\n\n";
+		}
 
 		// Now parse what we've got back
 		$this->message = new IXR_Message( wp_remote_retrieve_body( $response ) );