Index: src/wp-includes/class-http.php
===================================================================
--- src/wp-includes/class-http.php	(revision 38167)
+++ src/wp-includes/class-http.php	(working copy)
@@ -356,6 +356,7 @@
 			$options['proxy'] = new Requests_Proxy_HTTP( $proxy->host() . ':' . $proxy->port() );
 
 			if ( $proxy->use_authentication() ) {
+				$options['proxy']->use_authentication = true;
 				$options['proxy']->user = $proxy->username();
 				$options['proxy']->pass = $proxy->password();
 			}