Jax-RS Content-Type priority - rest

Question

I've got a JavaEE application with a simple REST service where I want to support the content types application/json and application/xml. Everything works as expected when the client sends an Accept header with one of these content types in it.
When this header is missing though, I want to answer with application/json as default, but no matter what I've tried so far, application/xml will be used.
Configuration Class
#ApplicationPath("api")
public class ApplicationConfig extends Application {
}
Service Class
#Path("users")
public class UserResource {
#Path("/{username}")
#GET
// The following variants didn't change the behaviour:
// #Produces({MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON})
// #Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML + ";q=0.5"})
// #Produces({MediaType.APPLICATION_JSON + ";q=1.0", MediaType.APPLICATION_XML + ";q=0.5"})
// #Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML + ";qs=0.5"})
#Produces({MediaType.APPLICATION_JSON, MediaType.APPLICATION_XML})
public User userDetails(#PathParam("username") final String username) {
return new User();
}
}
(Generated) User Class
The class I'm returning is generated (maven-jaxb2-plugin) with JAXB annotations to support XML without further configuration:
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name = "User", propOrder = {
"username"
})
#XmlRootElement(name = "user")
public class User
implements Serializable, Equals, HashCode
{
private final static long serialVersionUID = 1701L;
#XmlElement(required = true)
protected String username;
// ...
}
Further Information
JavaEE 7
Java 8
Same behaviour with Glassfish 4.1 and Wildfly 9.0.1