add feed_url property

The W3C feed validator recommends adding an atom:link tag with the URL
of the feed itself.

This adds a feed_url property to Channel which is used in an atom:link
tag.

Author: christianp

examples/simple-feed.php

@@ -17,6 +17,7 @@
     ->title('Channel Title')
     ->description('Channel Description')
     ->url('http://blog.example.com')
+    ->feed_url('http://blog.example.com/rss')
     ->language('en-US')
     ->copyright('Copyright 2012, Foo Bar')
     ->pubDate(strtotime('Tue, 21 Aug 2012 19:50:37 +0900'))

src/Suin/RSSWriter/Channel.php

@@ -14,6 +14,9 @@ class Channel implements ChannelInterface
     /** @var string */
     protected $url;
 
+    /** @var feed_url */
+    protected $feed_url;
+
     /** @var string */
     protected $description;
 
@@ -60,6 +63,17 @@ public function url($url)
         return $this;
     }
 
+    /**
+     * Set URL of this feed
+     * @param string $url
+     * @return $this;
+     */
+    public function feed_url($url)
+    {
+        $this->feed_url = $url;
+        return $this;
+    }
+
     /**
      * Set channel description
      * @param string $description
@@ -180,6 +194,13 @@ public function asXML()
         $xml->addChild('link', $this->url);
         $xml->addChild('description', $this->description);
 
+        if($this->feed_url !== null) {
+            $link = $xml->addChild('atom:link', '', "http://www.w3.org/2005/Atom");
+            $link->addAttribute('href',$this->feed_url);
+            $link->addAttribute('type','application/rss+xml');
+            $link->addAttribute('rel','self');
+        }
+
         if ($this->language !== null) {
             $xml->addChild('language', $this->language);
         }