code: def environment=(environment)
        environment = environment.try(:to_sym)
        if ENVIRONMENTS.include?(environment)
          @environment = environment
        else
          raise "Unknown environment '#{environment}'. Available: #{ENVIRONMENTS.join(', ')}"
        end
      end
docs: Configure with following options
   * :merchant_id (required)
   * :sign_key (defaults to false)
   * :environment (defaults to :development, available environments are defined in ENVIRONMENTS)
   * :proxy (a hash containing :http_proxyaddr, :http_proxyport, :http_proxyuser, :http_proxypass)