Don't move the timer_thread when it's enclosed

Don't move the timer_thread to ThreadGroup::Default, when it's
created in an enclosed ThreadGroup.
Prevents the exception: "add" can't move from the enclosed thread group"

lib/timeout.rb

@@ -120,7 +120,7 @@ def self.create_timeout_thread
         requests.reject!(&:done?)
       end
     end
-    ThreadGroup::Default.add(watcher)
+    ThreadGroup::Default.add(watcher) unless watcher.group.enclosed?
     watcher.name = "Timeout stdlib thread"
     watcher.thread_variable_set(:"\0__detached_thread__", true)
     watcher

test/test_timeout.rb

@@ -172,4 +172,24 @@ def test_threadgroup
     end;
   end
 
+  def test_handling_enclosed_threadgroup
+    # The problem "add: can't move from the enclosed thread group" #24,
+    # happens when the timeout_thread is created in an enclosed ThreadGroup.
+    # Make sure there isn't a timeout_thread in use to test this situation.
+    Timeout.instance_eval { @timeout_thread = nil }
+
+    t1 = Thread.new {
+      Thread.stop
+      assert_block do
+        Timeout.timeout(0.1) {}
+        true
+      end
+    }
+    sleep 0.1 while t1.status != 'sleep'
+    group = ThreadGroup.new
+    group.add(t1)
+    group.enclose
+    t1.run
+    t1.join
+  end
 end