[fix] Exception#dup: copy cause to the duplicated exception

Author: kares

core/src/main/java/org/jruby/RubyException.java

@@ -555,6 +555,7 @@ public void copySpecialInstanceVariables(IRubyObject clone) {
         RubyException exception = (RubyException)clone;
         exception.backtrace.copy(backtrace);
         exception.message = message;
+        exception.cause = cause;
     }
 
     /**