Added constant visibility

As we required PHP 7.1+ we can define visibility for constants.

Author: michalbundyra

docs/book/cookbook/setting-locale-depending-routing-parameter.md

@@ -152,7 +152,7 @@ use Psr\Http\Message\ServerRequestInterface;
 
 class LocalizationMiddleware implements MiddlewareInterface
 {
-    const LOCALIZATION_ATTRIBUTE = 'locale';
+    public const LOCALIZATION_ATTRIBUTE = 'locale';
 
     public function process(ServerRequestInterface $request, RequestHandlerInterface $handler) : ResponseInterface
     {

src/Handler/NotFoundHandler.php

@@ -17,8 +17,8 @@
 
 class NotFoundHandler implements RequestHandlerInterface
 {
-    const TEMPLATE_DEFAULT = 'error::404';
-    const LAYOUT_DEFAULT = 'layout::default';
+    public const TEMPLATE_DEFAULT = 'error::404';
+    public const LAYOUT_DEFAULT = 'layout::default';
 
     /**
      * @var TemplateRendererInterface

src/Middleware/ErrorResponseGenerator.php

@@ -17,7 +17,7 @@
 
 class ErrorResponseGenerator
 {
-    const TEMPLATE_DEFAULT = 'error::error';
+    public const TEMPLATE_DEFAULT = 'error::error';
 
     /**
      * @var bool

src/Middleware/ImplicitHeadMiddleware.php

@@ -45,7 +45,7 @@
  */
 class ImplicitHeadMiddleware implements MiddlewareInterface
 {
-    const FORWARDED_HTTP_METHOD_ATTRIBUTE = 'forwarded_http_method';
+    public const FORWARDED_HTTP_METHOD_ATTRIBUTE = 'forwarded_http_method';
 
     /**
      * @var null|ResponseInterface