Code around apparent bug in Thread#alive?

The documentation for Thread#alive? states that it "Returns true if [the thread] is running or sleeping." In ruby 1.9.3p429 (2013-05-15) [i386-mingw32], it also returns true when the status is 'aborting'. Explicitly checking for states fixes this bug.
Merge pull request #462 from cablegram/thread-alive

Code around apparent bug in Thread#alive?