[mruby] Leverage mruby-pack methods

[minami-o]

Hi there,
I would like to leverage the mruby-pack module using the mruby-handler directive.
My build says mruby-pack was added as an "included gem", but whenever I use the pack or unpack methods in a mruby-handler h2o.conf, I get an error like "undefined method 'unpack'".
Could anyone please help me out?
It feels like a really basic question, but everything I tried so far failed and I’m really new to mruby…
Thanks!