Feeds: `<comments>` is optional in RSS2, so don't include it when comments aren't present or open. Same for `<wfw:commentRss>` and `<slash:comments>`

Adds unit test.

Props swissspidy, realloc, hakre, sivel.
Fixes #9134.

Built from https://develop.svn.wordpress.org/trunk@35506


git-svn-id: http://core.svn.wordpress.org/trunk@35470 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/feed-atom.php

@@ -78,10 +78,13 @@ do_action( 'rss_tag_pre', 'atom' );
 	 * @since 2.0.0
 	 */
 	do_action( 'atom_entry' );
+
+	if ( get_comments_number() || comments_open() ) :
 		?>
 		<link rel="replies" type="<?php bloginfo_rss('html_type'); ?>" href="<?php the_permalink_rss() ?>#comments" thr:count="<?php echo get_comments_number()?>"/>
 		<link rel="replies" type="application/atom+xml" href="<?php echo esc_url( get_post_comments_feed_link(0, 'atom') ); ?>" thr:count="<?php echo get_comments_number()?>"/>
 		<thr:total><?php echo get_comments_number()?></thr:total>
+	<?php endif; ?>
 	</entry>
 	<?php endwhile ; ?>
 </feed>

wp-includes/feed-rss2.php

@@ -83,7 +83,9 @@ do_action( 'rss_tag_pre', 'rss2' );
 	<item>
 		<title><?php the_title_rss() ?></title>
 		<link><?php the_permalink_rss() ?></link>
+<?php if ( get_comments_number() || comments_open() ) : ?>
 		<comments><?php comments_link_feed(); ?></comments>
+<?php endif; ?>
 		<pubDate><?php echo mysql2date('D, d M Y H:i:s +0000', get_post_time('Y-m-d H:i:s', true), false); ?></pubDate>
 		<dc:creator><![CDATA[<?php the_author() ?>]]></dc:creator>
 		<?php the_category_rss('rss2') ?>
@@ -100,8 +102,10 @@ do_action( 'rss_tag_pre', 'rss2' );
 		<content:encoded><![CDATA[<?php the_excerpt_rss(); ?>]]></content:encoded>
 	<?php endif; ?>
 <?php endif; ?>
+<?php if ( get_comments_number() || comments_open() ) : ?>
 		<wfw:commentRss><?php echo esc_url( get_post_comments_feed_link(null, 'rss2') ); ?></wfw:commentRss>
 		<slash:comments><?php echo get_comments_number(); ?></slash:comments>
+<?php endif; ?>
 <?php rss_enclosure(); ?>
 	<?php
 	/**

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-beta2-35505';
+$wp_version = '4.4-beta2-35506';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.