Cache the association proxy object

This reimplements the behaviour of Rails 3, as I couldn't see why we
shouldn't cache the object, and @alindeman had a good use case for
caching it:

https://github.com/rails/rails/commit/c86a32d7451c5d901620ac58630460915292f88b#commitcomment-2784312

activerecord/lib/active_record/associations/collection_association.rb

@@ -34,7 +34,7 @@ def reader(force_reload = false)
           reload
         end
 
-        CollectionProxy.new(klass, self)
+        @proxy ||= CollectionProxy.new(klass, self)
       end
 
       # Implements the writer method, e.g. foo.items= for Foo.has_many :items

activerecord/test/cases/associations_test.rb

@@ -237,6 +237,11 @@ def test_scoped_allows_conditions
     assert david.projects.scope.is_a?(ActiveRecord::Relation)
     assert_equal david.projects, david.projects.scope
   end
+
+  test "proxy object is cached" do
+    david = developers(:david)
+    assert david.projects.equal?(david.projects)
+  end
 end
 
 class OverridingAssociationsTest < ActiveRecord::TestCase