Fix issue with restoring included attributes

Before this change the DispatcherServlet restored modified and also
removed added attributes but did not restore removed attributes.

Issue: SPR-10360

Author: rstoyanchev

spring-webmvc/src/main/java/org/springframework/web/servlet/DispatcherServlet.java

@@ -1268,6 +1268,7 @@ private void triggerAfterCompletionWithError(HttpServletRequest request, HttpSer
 	 * @param request current HTTP request
 	 * @param attributesSnapshot the snapshot of the request attributes before the include
 	 */
+	@SuppressWarnings("unchecked")
 	private void restoreAttributesAfterInclude(HttpServletRequest request, Map<?,?> attributesSnapshot) {
 		logger.debug("Restoring snapshot of request attributes after include");
 
@@ -1282,6 +1283,9 @@ private void restoreAttributesAfterInclude(HttpServletRequest request, Map<?,?>
 			}
 		}
 
+		// Add attributes that may have been removed
+		attrsToCheck.addAll((Set<String>) attributesSnapshot.keySet());
+
 		// Iterate over the attributes to check, restoring the original value
 		// or removing the attribute, respectively, if appropriate.
 		for (String attrName : attrsToCheck) {