`@JsonFormat(shape = Shape.POJO)` does not work for `java.util.Map` serialization via property annotation

(note: split off from #1419 )

So, although Class/default-config to serialize `Map`s works, doing the same via property annotation doesn't.
Following:

```
    static class Bean476Container
    {
        @JsonFormat(shape=JsonFormat.Shape.POJO)
        public Map<String, Object> stuff;
    }
```

will not use "POJO" style serialization.

