Topic: Rails server on IPv6

I am new to Ruby on Rails. I installed 3.0.18 on Cent OS 6 for testing.
I used following command to create a demo application
#rails new demo

Then goto this demo dir and use (without changing anything inside the
demo dir)
#rails server -p 80 -b ::
to make it listen on IPv4 and IPv6 both.

After that it works fine when using IPv4 address to access it. However,
when I using IPv6 address to send out POST request, it keeps giving me