#source returns an empty string instead of nil if no pages have been visited

This is more consistent with the behaviour of other methods, such as #current_url

lib/capybara/rack_test/browser.rb

@@ -92,7 +92,7 @@ class Capybara::RackTest::Browser
   def source
     last_response.body
   rescue Rack::Test::Error
-    nil
+    ""
   end
 
 protected