Fix ClassCastException to Nothing$ because of incorrectly infered type

When this code is ran ClassCastException gets thrown:
```java.lang.ClassCastException: org.scalatra.atmosphere.DefaultScalatraBroadcaster cannot be cast to scala.runtime.Nothing$```

This happens because [this](https://github.com/Atmosphere/atmosphere/commit/7e7f44579a75210a69055b8ec619214d2cf34618#diff-59efe98a5cb17cb15910bd0109763c02) change introduced Generic type inference in Java