Do not clone reason, let it up to exception

It was broken when reason was an exception class

Author: pitr-ch

lib/concurrent/promises.rb

@@ -1021,7 +1021,7 @@ def exception(*args)
           ex
         else
           ex = if reason[0].respond_to? :exception
-                 reason[0].clone.exception(*args)
+                 reason[0].exception(*args)
                else
                  RuntimeError.new(reason[0]).exception(*args)
                end