JSON-Jackson does not handle resource returning CompletionStage<X>

It seems like `jersey-media-json-jackson` is not up to date with the latest JAX-RS 2.1 features.
I tried creating the following resource method:

@GET
@Produces({"application/json" })
@Path("async")
public CompletionStage asyncHello(@NotNull @PathParam(value = "lang") String lang,
@NotNull @Size(min = 2) @QueryParam("yo") String greet){
return CompletableFuture.supplyAsync(() -> new Hello(lang, greet);
}

This fails with stack trace

```
java.lang.IllegalArgumentException: Class org.kantega.reststop.helloworld.jaxrs.Hello not subtype of [simple type, class java.util.concurrent.CompletionStage]
at com.fasterxml.jackson.databind.type.TypeFactory.constructSpecializedType(TypeFactory.java:359)
at org.glassfish.jersey.jackson.internal.jackson.jaxrs.base.ProviderBase.writeTo(ProviderBase.java:624)
at org.glassfish.jersey.message.internal.WriterInterceptorExecutor$TerminalWriterInterceptor.invokeWriteTo(WriterInterceptorExecutor.java:266)
```

Returning a plain String with `text/plain` works as expected.

Just as a test I build my own version of Jersey with some additional code in `org.glassfish.jersey.jackson.internal.jackson.jaxrs.base.ProviderBase.writeTo(...)`:

TypeFactory typeFactory = writer.getTypeFactory();

String typeName = genericType.getTypeName();
if(typeName.contains(CompletionStage.class.getName())) {
Type[] actualTypeArguments = ((ParameterizedTypeImpl) genericType).getActualTypeArguments();
genericType = actualTypeArguments[0];
}

JavaType baseType = typeFactory.constructType(genericType);

The middle part unwrapping the CompletionStage is new.
This works.

So, have I missed something, or is the `CompletionStage` support not complete?

Start with org.glassfish.jersey.jackson.internal.jackson.jaxrs.base.ProviderBase.writeTo(...) and reproduce the resource method returning CompletionStage with application/json. Compare the generic type handling with the plain String case; done means the Hello value is serialized successfully without the subtype exception.

Written by the indexing model from the issue text.