Do not modify frozen strings in place.

actionpack/lib/action_view/template.rb

@@ -119,7 +119,7 @@ def initialize(source, identifier, handler, details)
       @formats = Array.wrap(format).map(&:to_sym)
 
       @virtual_path = details[:virtual_path]
-      @virtual_path.sub!(".#{format}", "") if @virtual_path
+      @virtual_path.sub(".#{format}", "") if @virtual_path
     end
 
     def render(view, locals, &block)