Rewrite favicon.ico to the right location

This could happen if the server crashes using the web UI. The browser
sends a request to the /favicon.ico URL instead of
/core/img/favicon.ico, which is configured in the ownCloud's web pages.
Without this fix, the server responds with a 500 instead of a 200 or 404

lib/private/Setup.php

@@ -480,6 +480,7 @@ public static function updateHtaccess() {
 		if ($rewriteBase !== '') {
 			$content .= "\n<IfModule mod_rewrite.c>";
 			$content .= "\n  Options -MultiViews";
+			$content .= "\n  RewriteRule ^favicon.ico$ core/img/favicon.ico [L]";
 			$content .= "\n  RewriteRule ^core/js/oc.js$ index.php [PT,E=PATH_INFO:$1]";
 			$content .= "\n  RewriteRule ^core/preview.png$ index.php [PT,E=PATH_INFO:$1]";
 			$content .= "\n  RewriteCond %{REQUEST_FILENAME} !\\.(css|js|svg|gif|png|html|ttf|woff|ico|jpg|jpeg|json)$";