Fix MSVC "performance warning".

asio/include/asio/detail/task_io_service.hpp

diff --git a/asio/include/asio/detail/task_io_service.hpp b/asio/include/asio/detail/task_io_service.hpp
index 730ebe8..f7b4870 100644
--- a/asio/include/asio/detail/task_io_service.hpp
+++ b/asio/include/asio/detail/task_io_service.hpp
@@ -88,7 +88,7 @@
   // Return whether a handler can be dispatched immediately.
   bool can_dispatch()
   {
-    return thread_call_stack::contains(this);
+    return thread_call_stack::contains(this) != 0;
   }
 
   // Request invocation of the given handler.