add stale to response if cannot get enough credits

lib/paratika/response.rb

@@ -3,10 +3,12 @@ module Paratika
   class InvalidSignatureError < ::StandardError; end
   class InvalidResponseCodeError < ::StandardError; end
   class Response
+    STALE_ERROR_CODES = %w[ ERR20019 ERR20051 ]
     attr_reader :data
     def initialize(response)
       @data = JSON.parse(response.body).transform_keys!{ |k| snake_case(k) }.symbolize_keys
       @data.merge!({ time: DateTime.now, success: data[:response_code] == '00' })
+      @data.merge!({ stale: (STALE_ERROR_CODES.include? @data[:error_code]) }) unless @data[:success]
     end
 
     private