Adding the HEAD command

src/Client.php

@@ -114,6 +114,14 @@ public function body($article)
         return $this->sendCommand(new Command\BodyCommand($article));
     }
 
+    /**
+     * {@inheritdoc}
+     */
+    public function head($article)
+    {
+        return $this->sendCommand(new Command\HeadCommand($article));
+    }
+
     /**
      * {@inheritdoc}
      */

src/Command/HeadCommand.php

@@ -0,0 +1,79 @@
+<?php
+
+/*
+ * This file is part of the NNTP library.
+ *
+ * (c) Robin van der Vleuten <robin@webstronauts.co>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Rvdv\Nntp\Command;
+
+use Rvdv\Nntp\Exception\RuntimeException;
+use Rvdv\Nntp\Response\MultiLineResponse;
+use Rvdv\Nntp\Response\Response;
+
+/**
+ * HeadCommand.
+ *
+ * @author elabz
+ */
+class HeadCommand extends Command implements CommandInterface
+{
+    /**
+     * @var string
+     */
+    private $article;
+
+    /**
+     * Constructor.
+     *
+     * @param string $article
+     */
+    public function __construct($article)
+    {
+        $this->article = $article;
+
+        parent::__construct(true);
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function __invoke()
+    {
+        return sprintf('HEAD %s', $this->article);
+    }
+
+    /**
+     * @return string
+     */
+    public function onHeadFollows(MultiLineResponse $response)
+    {
+        return array_reduce($response->getLines(), function ($headers, $line) {
+            preg_match('/^([^\:]+)\:\s*(.*)$/', $line, $matches);
+            if (!empty($matches)) {
+                $headers[$matches[1]] = trim($matches[2]);
+            }
+
+            return $headers;
+        });
+    }
+
+    public function onNoNewsGroupCurrentSelected(Response $response)
+    {
+        throw new RuntimeException('A group must be selected first before getting an article header.', $response->getStatusCode());
+    }
+
+    public function onNoSuchArticleNumber(Response $response)
+    {
+        throw new RuntimeException('No article with that number.', $response->getStatusCode());
+    }
+
+    public function onNoSuchArticleId(Response $response)
+    {
+        throw new RuntimeException('No article with that message-id.', $response->getStatusCode());
+    }
+}

src/Response/Response.php

@@ -34,6 +34,7 @@ class Response implements ResponseInterface
         'GroupSelected' => 211, // rfc 3977
         'InformationFollows' => 215, // rfc 2980
         'ArticleFollows' => 220, //rfc 3977
+        'HeadFollows' => 221, //rfc 3977 Section 6.2.2
         'BodyFollows' => 222, //rfc 3977
         'OverviewInformationFollows' => 224, // rfc 2980
         'ArticleReceived' => 240, //rfc 3977