logger.rb: fix midnight log rotation miss

* lib/logger.rb (Logger::LogDevice#check_shift_log): compare the
  current time with the time for the next rotation to fix rotation
  miss when date changed between the comparison and log writing.
  based on the patch by megayu <yuhg2310 AT gmail.com>.
  [Fixes GH-539]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45071 b2dd03c8-39d4-4d8f-98ff-823fe69b080e