ob-ruby: clean up the session code some more and work around an I/O sync problem...

ob-ruby: clean up the session code some more and work around an I/O sync problem in comint

* lisp/ob-ruby.el (org-babel-ruby-evaluate): Clean up the session code
  and remove a superfluous `butlast'.  Work around a problem in comint
  where the first prompt in a session may be printed after the
  evaluation has already started, thus producing a spurious line in
  the output.

* testing/lisp/test-ob-ruby.el (test-ob-ruby/session-output-1,
  test-ob-ruby/session-output-2 test-ob-ruby/session-output-3): Test
  correct transfer of interpreter state across several session
  invocations.