Refactor redirect cache busting into middleware

docs/en/04_Changelogs/4.2.0.md

@@ -357,3 +357,7 @@ class PageController extends ContentController
    Note this is different from `Vary: Accept-Encoding`,
    which is important for compression (e.g. gzip), and usually added by
    other layers such as Apache's mod_gzip.
+ * DataObject no longer automatically invokes `HTTP::register_modification_date`
+   with LastModified on construction. User code should instead invoke
+   `HTTPCacheControlMiddleware::singleton()->registerModificationDate()` explicitly.
+   

src/Control/Middleware/HTTPCacheControlMiddleware.php

@@ -769,5 +769,10 @@ class HTTPCacheControlMiddleware implements HTTPMiddleware, Resettable
             // Even if publicCache(true) is specified, errors will be uncacheable
             $this->disableCache(true);
         }
+
+        // Don't cache redirects
+        if ($response->isRedirect()) {
+            $this->disableCache(true);
+        }
     }
 }

src/Control/RequestHandler.php

@@ -657,9 +657,6 @@ class RequestHandler extends ViewableData
      */
     public function redirectBack()
     {
-        // Don't cache the redirect back ever
-        HTTPCacheControlMiddleware::singleton()->disableCache(true);
-
         // Prefer to redirect to ?BackURL, but fall back to Referer header
         // As a last resort redirect to base url
         $url = $this->getBackURL()

src/Forms/Form.php

@@ -1812,13 +1812,9 @@ class Form extends ViewableData implements HasRequestHandler
     /**
      * Can the body of this form be cached?
      *
-     * Figure out if we can cache this form
-     * - forms with validation shouldn't be cached, because their error messages won't be shown
-     * - forms with security tokens shouldn't be cached because security tokens expire
-     *
      * @return bool
      */
-    public function canBeCached()
+    protected function canBeCached()
     {
         if ($this->getSecurityToken()->isEnabled()) {
             return false;