Avoid having to store complex object in the default translation file

This make possible for applications to use the safe mode of yaml to
load the translations files.

Fixes #41459.

activesupport/lib/active_support/locale/en.yml

@@ -45,7 +45,7 @@ en:
       # Number of decimals, behind the separator (the number 1 with a precision of 2 gives: 1.00)
       precision: 3
       # Determine how rounding is performed (see BigDecimal::mode)
-      round_mode: !ruby/sym default
+      round_mode: default
       # If set to true, precision will mean the number of significant digits instead
       # of the number of decimal digits (1234 with precision 2 becomes 1200, 1.23543 becomes 1.2)
       significant: false

activesupport/lib/active_support/number_helper/rounding_helper.rb

@@ -13,7 +13,7 @@ def round(number)
         precision = absolute_precision(number)
         return number unless precision
 
-        rounded_number = convert_to_decimal(number).round(precision, options.fetch(:round_mode, :default))
+        rounded_number = convert_to_decimal(number).round(precision, options.fetch(:round_mode, :default).to_sym)
         rounded_number.zero? ? rounded_number.abs : rounded_number # prevent showing negative zeros
       end