Add configure() support for issues

Borrowed from issue comments, but changed the default bodyType to "raw" since the documentation indicates that is the default when no specific media type is passed: https://developer.github.com/v3/media/#comment-body-properties

Author: bobeagan

lib/Github/Api/Issue.php

@@ -18,6 +18,22 @@
  */
 class Issue extends AbstractApi
 {
+    use AcceptHeaderTrait;
+
+    /**
+     * Configure the body type.
+     *
+     * @link https://developer.github.com/v3/issues/#custom-media-types
+     * @param string|null $bodyType
+     */
+    public function configure($bodyType = null)
+    {
+        if (!in_array($bodyType, array('text', 'html', 'full'))) {
+            $bodyType = 'raw';
+        }
+        $this->acceptHeaderValue = sprintf('application/vnd.github.%s.%s+json', $this->client->getApiVersion(), $bodyType);
+    }
+
     /**
      * List issues by username, repo and state.
      *