Return the particular thread for Thread#kill.

This way we could write something like this in one line:

    Thread.new{}.kill.join

This is how MRI behaves:

    $ ruby -ve 'p Thread.new{}.kill'
    ruby 2.0.0p247 (2013-06-27 revision 41674) [x86_64-darwin12.2.1]
    #<Thread:0x007fdb2494a898 run>
Spec for Thread.kill. It should return the thread it is killing.
Merge pull request #2677 from godfat/Thread.kill-returns-the-thread

Return the particular thread for Thread#kill.