Invoke {blacklisted,throttled}_response with #call

I have a response which is a class. While I can still have my class
implement `#[]`, it does look a bit off. On the other side, having
objects, responding to #call, that are not procs is pretty common.

So I propose to invoke the responses with `#call` to let users override
it with response objects, that respond to `#call` instead of `#[]`.

Author: gsamokovarov

README.md

@@ -73,9 +73,9 @@ def call(env)
   if whitelisted?(req)
     @app.call(env)
   elsif blacklisted?(req)
-    blacklisted_response[env]
+    self.class.blacklisted_response.call(env)
   elsif throttled?(req)
-    throttled_response[env]
+    self.class.throttled_response.call(env)
   else
     tracked?(req)
     @app.call(env)

lib/rack/attack.rb

@@ -96,9 +96,9 @@ def call(env)
     if whitelisted?(req)
       @app.call(env)
     elsif blacklisted?(req)
-      self.class.blacklisted_response[env]
+      self.class.blacklisted_response.call(env)
     elsif throttled?(req)
-      self.class.throttled_response[env]
+      self.class.throttled_response.call(env)
     else
       tracked?(req)
       @app.call(env)