Fixes #5608. Kernel#warn doesn't handle exceptions as expected.

This was a more basic problem that was not calling to_s() in the case that
the argument is not already a String.

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

@@ -1244,7 +1244,7 @@ public static IRubyObject warn(ThreadContext context, IRubyObject recv, IRubyObj
             return context.nil;
         }
 
-        return warn(context, recv, _message.convertToString());
+        return warn(context, recv, _message.asString());
     }
 
     static IRubyObject warn(ThreadContext context, IRubyObject recv, RubyString message) {