Add 1 second buffer to expiry to correct throttles

Fixes #69.

There was a race condition when `Time.now.to_i` changes between when
`epoch_time` is computed in line 18, and the cache request is made (and
the `key` is expired).

I.e., a throttle check starts at t0, but doesn’t reach the cache until
t1, the cache will have expired the throttle count. The request will
likely be allowed, even if the request exceeded the limit.

This has the effect of keeping keys in cache about 1 second longer than
strictly necessary. But the extra cache space seems like a good
trade-off for correct throttling.

lib/rack/attack/cache.rb

@@ -16,7 +16,8 @@ module Rack
 
       def count(unprefixed_key, period)
         epoch_time = Time.now.to_i
-        expires_in = period - (epoch_time % period)
+        # Add 1 to expires_in to avoid timing error: http://git.io/i1PHXA
+        expires_in = period - (epoch_time % period) + 1
         key = "#{prefix}:#{(epoch_time/period).to_i}:#{unprefixed_key}"
         do_count(key, expires_in)
       end