StyleSheet/JavaScript: Also include Cache-Control when answering with a 304

Without Cache-Control, the browser may invalidate its cached file and we're
serving the file again on the next request.

refs #2917

library/Icinga/Web/JavaScript.php

@@ -98,13 +98,13 @@ class JavaScript
         $request = Icinga::app()->getRequest();
         $noCache = $request->getHeader('Cache-Control') === 'no-cache' || $request->getHeader('Pragma') === 'no-cache';
 
+        header('Cache-Control: public');
         if (! $noCache && FileCache::etagMatchesFiles($files)) {
             header("HTTP/1.1 304 Not Modified");
             return;
         } else {
             $etag = FileCache::etagForFiles($files);
         }
-        header('Cache-Control: public');
         header('ETag: "' . $etag . '"');
         header('Content-Type: application/javascript');
 

library/Icinga/Web/StyleSheet.php

@@ -182,6 +182,7 @@ class StyleSheet
 
         $request = $styleSheet->app->getRequest();
         $response = $styleSheet->app->getResponse();
+        $response->setHeader('Cache-Control', 'public', true);
 
         $noCache = $request->getHeader('Cache-Control') === 'no-cache' || $request->getHeader('Pragma') === 'no-cache';
 
@@ -194,9 +195,7 @@ class StyleSheet
 
         $etag = FileCache::etagForFiles($styleSheet->lessCompiler->getLessFiles());
 
-        $response
+        $response->setHeader('ETag', $etag, true)
-            ->setHeader('Cache-Control', 'public', true)
-            ->setHeader('ETag', $etag, true)
             ->setHeader('Content-Type', 'text/css', true);
 
         $cacheFile = 'icinga-' . $etag . ($minified ? '.min' : '') . '.css';