The problem is your use of the @RequestScoped
annotation on the producer method. Remove it and the application will work as expected.
The Request Scoped annotation is used to annotate Beans managed by the container. To do so, the container proxies the object's public methods. Final classes like String are however not proxyable, as pointed out by the exception when running the code on Glassfish 4.0 with Weld 2.0.0 SP1:
WELD-001437 Normal scoped bean class java.lang.String is not proxyable because the type is final or it contains a final method class java.lang.String - Producer Method [String] (...etc.)