Fix Rack response for Puma webserver

* Rack expects an enumerable. An empty string doesn't work (it worked for Thin, though).

Author: jochenehret

lib/cloud_controller/metrics_webserver.rb

@@ -5,7 +5,6 @@ module VCAP
   module CloudController
     class MetricsWebserver
       attr_reader :app
-      @server
 
       def initialize
         @app = build_app

middleware/cors.rb

@@ -33,7 +33,8 @@ def call(env)
                                    })
         end
 
-        return [200, preflight_headers, ''] if env['REQUEST_METHOD'] == 'OPTIONS'
+        # NOTE: The response body must be an enumerable for Puma. Returning an empty string does not work.
+        return [200, preflight_headers, ['']] if env['REQUEST_METHOD'] == 'OPTIONS'
 
         status, headers, body = call_app(env)