ShowDocSpec: add a test concerning monkeypatching

Make sure that `show-doc` displays the comments of the original
definition by default. This test *must* fail with current state of
things. The goal is to eliminate the oddities in Pry across some Ruby
implementations.

For more information see:
https://github.com/pry/pry/commit/d05233ecd3ea0bba4c6fc0fb5c932631db687383