Clear cache while adding a new feed

Before, the feed cache was preventing to add a feed when using twice the
bookmarklet if the first time the user aborted the process.
Now, the cache is removed for the feed before loading information. This
behavior as a limited impact since only the selected feed cache is removed.

See #3658

app/Controllers/feedController.php

@@ -218,6 +218,7 @@ class FreshRSS_feed_Controller extends Minz_ActionController {
 			$this->view->feed = new FreshRSS_Feed($url);
 			try {
 				// We try to get more information about the feed.
+				$this->view->feed->_attributes('clear_cache', true);
 				$this->view->feed->load(true);
 				$this->view->load_ok = true;
 			} catch (Exception $e) {