[Fixes #253] Avoid 'defined?' buggy behavior in ruby 2.5.0

[grzuy]

Fixes #253

Found that defined? is buggy in ruby 2.5.0, which under certain circumstances users using rack-attack can hit (see minimal app to reproduce issue 253).

I already reported (https://bugs.ruby-lang.org/issues/14407) and fixed (ruby/ruby#1800) the underlaying issue in ruby, but i am guessing it's going to take some time before there's a new ruby release including the fix.

So for now, we would need to circumvent this ruby 2.5.0 bug by using Object#const_defined? instead of defined?, at least for this particular case.

---

More details:

Anyone using:

• ruby 2.5.0
• redis
• rack-attack without redis-store and using at least one throttle
• having a toplevel class named Store

will hit this ruby 2.5.0 bug https://bugs.ruby-lang.org/issues/14407

That's because of the following buggy behavior of defined? under ruby 2.5.0:

$ ruby -v
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-linux]

$ irb
> class Redis
> end
=> nil
> class Store
> end
=> nil
> defined?(::Redis::Store)
=> "constant"
> ::Redis::Store
  NameError (uninitialized constant Redis::Store
    Did you mean?  Store)

[grzuy]

For anyone interested in testing it, here is a minimal ruby app that reproduces the issue: https://github.com/grzuy/ruby_app_for_rack_attack_253.

In that app's master branch you can reproduce the issue.
In the fix_attempt branch, which is setting rack-attack's dependency to this PR's branch, you can test that the issue is gone :-)

[grzuy]

@frewsxcv Do you have any feedback about this one? :-)