Prefer Kernel#sprintf over String#%

String#% takes the arguments as an Array object when passing in multiple values,
which requires us to create an extra object.

railties/lib/rails/rack/logger.rb

@@ -46,11 +46,11 @@ def call_app(request, env) # :doc:
 
         # Started GET "/session/new" for 127.0.0.1 at 2012-09-26 14:51:42 -0700
         def started_request_message(request) # :doc:
-          'Started %s "%s" for %s at %s' % [
+          sprintf('Started %s "%s" for %s at %s',
             request.raw_request_method,
             request.filtered_path,
             request.remote_ip,
-            Time.now.to_default_s ]
+            Time.now.to_default_s)
         end
 
         def compute_tags(request) # :doc: