Fix that built-in error message response not HTML escaped.

Stack traces and Object#inspect output lots of less-than signs
that need escaping.

There's probably a better way to do this. My first iteration called
CGI::escapeHTML and then I noticed that Rack includes a nice Utils module with
various escaping methods so I figured that would be a better fit for Sinatra.
Unfortunately, these methods don't seem to be exposed anywhere (that I could
find) so I extended the Rack::Utils module with itself so that messages could be
sent directly to the module.

lib/sinatra.rb

@@ -44,6 +44,10 @@ module Rack #:nodoc:
     
   end
   
+  module Utils
+    extend self
+  end
+
 end
 
 module Sinatra
@@ -623,8 +627,8 @@ module Sinatra
           		<div id="content">
             		<img src="/sinatra_custom_images/500.png" />
           			<div id="stacktrace">
-          				<h1>#{@error.message}</h1>
-          				<pre><code>#{@error.backtrace.join("\n")}</code></pre>
+          				<h1>#{Rack::Utils.escape_html(@error.message)}</h1>
+          				<pre><code>#{Rack::Utils.escape_html(@error.backtrace.join("\n"))}</code></pre>
           		</div>
           	</body>
           </html>