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merges r31623 from trunk into ruby_1_9_2.¶

• lib/timeout.rb (Timeout#timeout): don't leak "execution expired"
  exception. [Bug #4283] [ruby-core:34534].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_2@31837 b2dd03c8-39d4-4d8f-98ff-823fe69b080e