Add ActionView::OutputBuffer#concat back

Fix: rails#45777

The method is being used by HAML, it's easy to add it
back.

Author: byroot

actionview/lib/action_view/buffers.rb

@@ -50,6 +50,7 @@ def <<(value)
       end
       self
     end
+    alias :concat, :<<
     alias :append= :<<
 
     def safe_concat(value)