`Response#charset=` uses `default_charset` when `nil` is passed

actionpack/lib/action_dispatch/http/response.rb

@@ -252,7 +252,7 @@ def sending_file=(v)
     end
 
     # Sets the HTTP character set. In case of +nil+ parameter
-    # it sets the charset to utf-8.
+    # it sets the charset to +default_charset+.
     #
     #   response.charset = 'utf-16' # => 'utf-16'
     #   response.charset = nil      # => 'utf-8'