Default to test mode in test or development.

Unless configured otherwise, Rakismet will default to being in test mode
when the current `Rails.env` is `test` or `development`. Of course any
explicit configuration via `Rakismet.test = true|false` will override
this default.

lib/rakismet/railtie.rb

@@ -14,6 +14,7 @@ class Railtie < Rails::Railtie
         Rakismet.host = app.config.rakismet[:host]
         Rakismet.proxy_host = app.config.rakismet[:proxy_host]
         Rakismet.proxy_port = app.config.rakismet[:proxy_port]
+        Rakismet.test = app.config.rakismet.fetch(:test) { Rails.env.test? || Rails.env.development? }
         app.middleware.use Rakismet::Middleware if app.config.rakismet.use_middleware
       end