title: Rspec let variable not working with have_title
body: <p>I'm having trouble getting one of my Rspec tests to pass using Capybara 2.1.0</p>
  
  <p>This works: </p>
  
  <pre><code>require 'spec_helper'
  
  describe "Static pages" do
  
    describe "Home page" do
  
        it "should have the title 'Home'" do
        visit '/static_pages/home'
        page.should have_title("MyApp | Home")
      end
    end
  end
  </code></pre>
  
  <p>This does not</p>
  
  <pre><code>require 'spec_helper'
  
  describe "Static pages" do
  
    let(:site_title) {"MyApp"}
  
    describe "Home page" do
  
        it "should have the title 'Home'" do
        visit '/static_pages/home'
        page.should have_title("#{site_title} | Home")
      end
    end
  end
  </code></pre>
  
  <p>Any thoughts? I've been working on it for quite a while. Here is the error message that I receive. </p>
  
  <pre><code>1) Static pages Home page should haven the title
   Failure/Error: page.should have_title("#{site_title} | Home")
     expected #has_title?("MyApp | Home") to return true, got false
   # ./spec/requests/static_pages_spec.rb:16:in `block (3 levels) in &lt;top (required)&gt;'
  </code></pre>