respond_to? needs include_private argument

respond_to? needs include_private argument

Sup started mysteriously bailing out on me today after an apt-get
update with stuff like this:

 --- ArgumentError from thread: main
wrong number of arguments (2 for 1)
/usr/lib/ruby/1.8/sup/index.rb:424:in `respond_to?'
/usr/lib/ruby/1.8/sup/index.rb:424:in `flatten'
/usr/lib/ruby/1.8/sup/index.rb:424:in `load_sources'
/usr/lib/ruby/1.8/sup/index.rb:108:in `load'
/usr/lib/ruby/1.8/sup/util.rb:497:in `send'
/usr/lib/ruby/1.8/sup/util.rb:497:in `method_missing'
/usr/bin/sup:122

I did some digging and it seems like when Array#flatten, in attempting
to figure out if it can flatten some list element recursively, sends
it :respond_to? with both arguments -- I didn't even know there was a
second one (defaults to false; see docs). But this only happened recently
or something.

Here's somewhere else this came up:
http://www.ruby-forum.com/topic/154938

So, the fix is exactly the same. Should work fine on old Ruby as well.