End type lost when using promise()

The Traversal.promise method loses type information when used so the type of the returned future is not as complete as it could be.

For example:

g.V().hasLabel("person").has("name", "marko").promise(Traversal::next);

Returns a CompletableFuture<Object> instead of CompletableFuture<Vertex>.

g.V().hasLabel("person").promise(Traversal::toList);

Returns a CompletableFuture<List> instead of CompletableFuture<List<Vertex>.

The fix for this could be as simple as adding the start and end (S,E) types to the traversal that is the input of the function:

public default <T> CompletableFuture<T> promise(final Function<Traversal, T> traversalFunction)

to:

public default <T> CompletableFuture<T> promise(final Function<Traversal<S,E>, T> traversalFunction)

Need to test this a bit more though, but I think that should work.