Add tests from Rubyspec and more methods for Ruby wrappers

Hi Arturaz,

I'm preparing a talk for JRuby <-> Scala interoperability and I've added some stuff to your nicely written gem.
I'm using Rubyspec to test that the Ruby Map wrapper behaves like a normal Hash.
It would be great if you could have a look and accept the pull request if it's fine with you :)

Best
  Martin