Use new net/smtp in ruby 1.8.5 (SMTPS support)

I’d like to use the new net/smtp library in ruby 1.8.5 because it
supports SMTPS.

What should I do in order to install just what is needed and nothing
Which is the right way (if any) to make the new library cohexist the
old one? I mean, so that I can use it in a project, but not as a
general replacement of 1.8.5 version? Or should I replace the entire
existing net library?

I’m using 1.8.4 on Windows (one click installer) and 1.8.5 on MacOSX
(darwinports), but I may easily upgrade on 1.8.5 on Windows as well if
it is crucial.