code: def query_string(params, prefix = nil) #:nodoc:
        prefix = "#{prefix}." if prefix
        params.keys.sort { |a, b| a.to_s <=> b.to_s }.collect do |key|
          case val = params[key]
            when Hash
            query_string(val, key)
            else
            "#{prefix}#{key}=#{escape val}"
          end
        end.join('&')
      end
docs: Flattens a possibly-nested hash into a query string for Amazon.
   With Amazon, nested hashes are flattened with a period, as follows:
  
     AmazonFlexPay::Util.query_string(:foo => {:hello => 'world'})
     => "foo.hello=world"