Reset configuration when Rails reloads.

I feel like I've hit this problem before, and I thought I'd restructured things in v3 onwards to avoid it, but I guess not. It's reared its head again in #1125 - the cached configuration's indices kept onto their old references to models, whereas models returned from ActiveRecord search calls were using the new references, and thus not quite matching. It wasn't a problem if all models were using the default primary key of 'id', but otherwise with alternative primary keys things would break.

lib/thinking_sphinx/railtie.rb

@@ -1,6 +1,10 @@
 # frozen_string_literal: true
 
 class ThinkingSphinx::Railtie < Rails::Railtie
+ config.to_prepare do
+ ThinkingSphinx::Configuration.reset
+ end
+
  initializer 'thinking_sphinx.initialisation' do
  ActiveSupport.on_load(:active_record) do
  ActiveRecord::Base.send :include, ThinkingSphinx::ActiveRecord::Base