Size reports incorrectly for collections when more than one item is added with build

Adding more than one item to a collection using collection.build and then subsequently checking the size of the collection with the size method fails to work correctly.

This code from active_record/associations/association_collection.rb:

unsaved_records = Array(@target.detect { |r| r.new_record? })

unsaved_records.size + count_records

should read:

unsaved_records = Array(@target.select { |r| r.new_record? })

unsaved_records.size + count_records

I attach a git diff for patching the code and adding a test for this.