[2.2.1-java] impossible to use promise with withTransaction into @Transactional controllers

Example: https://github.com/zadonskiyd/play2jpaProblemExample

```java
F.Promise<String> helloPromise = promise(new F.Function0<String>() {
    @Override
    public String apply() throws Throwable {
        return JPA.withTransaction(new F.Function0<String>() {
            @Override
            public String apply() throws Throwable {
                return "hello";
             }
        });
    }
});
String hello = helloPromise.get(1000);
```
removes em from current thread ?
```
[RuntimeException: No EntityManager bound to this thread. 
Try to annotate your action method with @play.db.jpa.Transactional] 
```
at
```
84        Long total = (Long)JPA.em()
```

In 2.1.0 same code (with Akka.future instead of promise) works fine