You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: referrer_policy.rb
    path of file: ./repos/secure_headers/lib/secure_headers/headers
    
    the code of the file until where you have to start completion: # frozen_string_literal: true
module SecureHeaders
  class ReferrerPolicyConfigError < StandardErro
    
      def validate_config!(config)
        case config
        when nil, OPT_OUT
          # valid
        when String, Array
          config = Array(config)
          unless config.all? { |t| t.is_a?(String) && VALID_POLICIES.include?(t.downcase) }
            raise ReferrerPolicyConfigError.new("Value can only be one or more of #{VALID_POLICIES.join(", ")}")
          end