Feeds: Replace the RSS2 `lastBuildDate` date field with the `r` date specifier.

See #39141.

Built from https://develop.svn.wordpress.org/trunk@39614


git-svn-id: http://core.svn.wordpress.org/trunk@39554 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/feed-rss2.php

@@ -44,7 +44,7 @@ do_action( 'rss_tag_pre', 'rss2' );
 	<description><?php bloginfo_rss("description") ?></description>
 	<lastBuildDate><?php
 		$date = get_lastpostmodified( 'GMT' );
-		echo $date ? mysql2date( 'D, d M Y H:i:s +0000', $date, false ) : date( 'D, d M Y H:i:s +0000' );
+		echo $date ? mysql2date( 'r', $date, false ) : date( 'r' );
 	?></lastBuildDate>
 	<language><?php bloginfo_rss( 'language' ); ?></language>
 	<sy:updatePeriod><?php

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8-alpha-39613';
+$wp_version = '4.8-alpha-39614';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.