title: Rails / RSpec: How to trigger Puffing Billy only on specific tests by using RSpec metadata
body: <p>I would like to switch to <code>webkit_billy</code> (the Javascript driver of <a href="https://github.com/oesmith/puffing-billy/" rel="nofollow noreferrer">Puffing Billy gem</a>)  <strong>only in certain tests</strong>.  I want to be able to use:</p>
  
  <pre><code>describe "xxx", billy: true do
  end
  </code></pre>
  
  <p>This should signal to RSpec that I want to switch to puffing billy driver.</p>
  
  <p>I wrote this in <code>spec_helper.rb</code> but it's not working:</p>
  
  <pre><code>  config.before(:each) do |example|
      if [:billy].include? example.metadata[:type]
        Capybara.current_driver     = :webkit_billy
        Capybara.javascript_driver  = :webkit_billy
      else
        Capybara.current_driver     = :webkit
        Capybara.javascript_driver  = :webkit 
      end
    end
  </code></pre>