Avoid 'defined?' buggy behavior in ruby 2.5.0. Fixes rack#253

'defined?' is buggy in ruby 2.5.0, which under certain circumstances
users using rack-attack can hit. See issue rack#253.

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

So for now we would need to circumvent this bug by using
'const_defined?' instead of 'defined?' 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:

```
$ 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)
```

lib/rack/attack/store_proxy/redis_store_proxy.rb

@@ -5,7 +5,14 @@ class Attack
     module StoreProxy
       class RedisStoreProxy < SimpleDelegator
         def self.handle?(store)
-          defined?(::Redis::Store) && store.is_a?(::Redis::Store)
+          # Using const_defined? for now.
+          #
+          # Go back to use defined? once this ruby issue is
+          # fixed and released:
+          # https://bugs.ruby-lang.org/issues/14407
+          #
+          # defined?(::Redis::Store) && store.is_a?(::Redis::Store)
+          const_defined?("::Redis::Store") && store.is_a?(::Redis::Store)
         end
 
         def initialize(store)