Fix `while do` double indents (#37)

`@cond_stack` is used to detect if a `do` is optional or not.

  while true do # <- This `do` is optional, and does not
                #    increment the level of indentation.
  end

`@cond_stack` is basically equivalent to `cond_stack` in
parse.y of MRI, but my implementation mistakes when `until`,
`while` or `for ... in` appear in a condition of `until` and
the rest, like this:

  while until 1 do
          :until_body
        end do # <- ERM thinks this `do` is not optional.
    :while_body
  end

I've once fixed this in
https://github.com/vzvu3k6k/enhanced-ruby-mode/commit/6e6dc90d2808513232d82807dde7f93b456ab60f
but I postponed merging this because
* I think few want to write such a code, and I'm afraid that
  it adds bigger complexity than the problem it solves.
* As ERM thinks the `until` in the second sample code is modifier,
  the second sample is not indented properly
  whether optional `do`s are correctly detected or not.

FYI: To understand how MRI handles with `while do`,
"The `do` conflict" section in chapter 11 of Ruby Hacking Guide
(http://ruby-hacking-guide.github.io/contextual.html) is
helpful.