[ruby/csv] Resolve CSV::Converters and HeaderConverters lazy

It's for Ractor. If you want to use the built-in converters, you
should call Ractor.make_shareable(CSV::Converters) and/or
Ractor.make_shareable(CSV::HeaderConverters).

ruby/csv@b0b1325d6b

lib/csv.rb

@@ -2575,7 +2575,7 @@ def parser_fields_converter
 
   def build_parser_fields_converter
     specific_options = {
-      builtin_converters: Converters,
+      builtin_converters_name: :Converters,
     }
     options = @base_fields_converter_options.merge(specific_options)
     build_fields_converter(@initial_converters, options)
@@ -2587,7 +2587,7 @@ def header_fields_converter
 
   def build_header_fields_converter
     specific_options = {
-      builtin_converters: HeaderConverters,
+      builtin_converters_name: :HeaderConverters,
       accept_nil: true,
     }
     options = @base_fields_converter_options.merge(specific_options)

lib/csv/fields_converter.rb

@@ -16,15 +16,15 @@ def initialize(options={})
       @empty_value = options[:empty_value]
       @empty_value_is_empty_string = (@empty_value == "")
       @accept_nil = options[:accept_nil]
-      @builtin_converters = options[:builtin_converters]
+      @builtin_converters_name = options[:builtin_converters_name]
       @need_static_convert = need_static_convert?
     end
 
     def add_converter(name=nil, &converter)
       if name.nil?  # custom converter
         @converters << converter
       else          # named converter
-        combo = @builtin_converters[name]
+        combo = builtin_converters[name]
         case combo
         when Array  # combo converter
           combo.each do |sub_name|
@@ -80,5 +80,9 @@ def need_convert?
       @need_static_convert or
         (not @converters.empty?)
     end
+
+    def builtin_converters
+      @builtin_converters ||= ::CSV.const_get(@builtin_converters_name)
+    end
   end
 end