Decreasing header values cause an exception.

[tenderlove]

If you have a wiki page that starts at header level 3, then goes to 2, then 1, you get an exception on the level 1 header.

Here's an example:

require 'wikicloth'

WikiCloth::WikiCloth.new(:data => DATA.read)

__END__
=== X ===

== Y ==

= Z =

If I run the above program, I get this exception:

[aaron@TC wikicloth (master)]$ ruby -I lib test.rb 
/Users/aaron/github/repos/wikicloth/lib/wikicloth.rb:42:in `block in load': undefined method `<<' for nil:NilClass (NoMethodError)
    from /Users/aaron/github/repos/wikicloth/lib/wikicloth.rb:37:in `each_line'
    from /Users/aaron/github/repos/wikicloth/lib/wikicloth.rb:37:in `load'
    from /Users/aaron/github/repos/wikicloth/lib/wikicloth.rb:26:in `initialize'
    from test.rb:3:in `new'
    from test.rb:3:in `<main>'
[aaron@TC wikicloth (master)]$

I'd expect it to output h3, then h2, then h1.

Thanks!