JRuby class and Java Scripting Engine unexpected results

App.java

import javax.script.Invocable;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;

/*
 * http://stackoverflow.com/questions/10692951/jruby-class-and-java-scripting-engine-unexpected-results
 */
public class App {

    public static void main( String[] args ) throws Exception{
        ScriptEngineManager sm = new ScriptEngineManager();
        ScriptEngine se = sm.getEngineByName("jruby");

        StringBuilder sb = new StringBuilder();
        sb.append("class Test\n");
        sb.append("   attr_accessor :a, :b\n");
        sb.append("   def str\n");
        sb.append("      \"#{a}, #{b} is a test.\"\n");
        sb.append("   end\n");
        sb.append("end\n");
        sb.append("o = Test.new\n");
        sb.append("o.a = 'A'\n");
        sb.append("o.b = 'B'\n");
        sb.append("o");

        Object o = se.eval(sb.toString());

        System.out.println( ((Invocable) se).invokeMethod(o, "str"));
    }
}