Escape HTML in the 404 page.

There is a reflected XSS in the development mode 404 page for clients
that don't URL-encode the request path. (I'm not aware of any major
browsers that do this, but you can see the idea with cURL.)

lib/sinatra/base.rb

@@ -1941,7 +1941,7 @@ module Sinatra
             <img src='#{uri "/__sinatra__/404.png"}'>
             <div id="c">
               Try this:
-              <pre>#{code}</pre>
+              <pre>#{Rack::Utils.escape_html(code)}</pre>
             </div>
           </body>
           </html>