Ruby

Logger: simple refactoring

• lib/logger.rb (Logger#level=): remove unnecessary local
  variable.
• lib/logger.rb (Logger#initialize, Logger#reopen): [DOC] mention
  the default values. cherrypicked from [GH-1319].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@54497 b2dd03c8-39d4-4d8f-98ff-823fe69b080e