Associations: #decorate queries DB every time

`ActiveRecord::Associations::CollectionProxy#decorate` ignores `target`
(be it already loaded or not) and loads the association again every time
it's called.

That's why unsaved records get missing from the decorated collection.

Resolves drapergem#646,
         drapergem#706,
         drapergem#827.

lib/draper/decoratable.rb

@@ -12,8 +12,12 @@ module Decoratable
     extend ActiveSupport::Concern
     include Draper::Decoratable::Equality
 
+    autoload :CollectionProxy, 'draper/decoratable/collection_proxy'
+
     included do
       prepend Draper::Compatibility::Broadcastable if defined? Turbo::Broadcastable
+
+      ActiveRecord::Associations::CollectionProxy.include CollectionProxy if defined? ActiveRecord
     end
 
     # Decorates the object using the inferred {#decorator_class}.

lib/draper/decoratable/collection_proxy.rb

@@ -0,0 +1,15 @@
+module Draper
+  module Decoratable
+    module CollectionProxy
+      # Decorates a collection of objects. Used at the end of a scope chain.
+      #
+      # @example
+      #   company.products.popular.decorate
+      # @param [Hash] options
+      #   see {Decorator.decorate_collection}.
+      def decorate(options = {})
+        decorator_class.decorate_collection(load_target, options.reverse_merge(with: nil))
+      end
+    end
+  end
+end