within to send new_scope on yield (1.1)

Hi,

(Same as #1076 but for 1.1 because I am not sure yet we can upgrade to 2.0 easily)

because I need the scope to write an rspec matcher such as ....

```ruby
    within(level_selector) do |scope|
        expect(scope.text).to have_content(header_text)
    end
```

the only solution I found was to add this to capybara.

thanks