Find elements with no locator

Currently the following page object will throw error:

```ruby
class Page
  include PageObject

  spans :test
end

page = Page.new
page.test_elements
#=> lib/page-object/accessors.rb:1202:in `clone': can't clone NilClass (TypeError)
```

Quick fix for this. Sorry, I haven't found a good place to add spec, but can easily do it if you point me to correct file.