logger.rb: simplify

* lib/logger.rb (format_datetime): simplify and freeze the default
  format so that it can be shared.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47272 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/logger.rb

@@ -510,11 +510,7 @@ private
   private
 
     def format_datetime(time)
-      if @datetime_format.nil?
-        time.strftime("%Y-%m-%dT%H:%M:%S.%6N ")
-      else
-        time.strftime(@datetime_format)
-      end
+      time.strftime(@datetime_format || "%Y-%m-%dT%H:%M:%S.%6N ".freeze)
     end
 
     def msg2str(msg)