[ruby/timeout] Synchronize all accesses to @done

* So it is trivially correct.
* Performance seems the same overall.

5e0d8e1637

lib/timeout.rb

@@ -67,12 +67,14 @@ module Timeout
       @message = message
 
       @mutex = Mutex.new
-      @done = false
+      @done = false # protected by @mutex
     end
 
     def done?
+      @mutex.synchronize do
         @done
       end
+    end
 
     def expired?(now)
       now >= @deadline and !done?