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

[cowtowncoder]

(note: split off from #1419 )

So, although Class/default-config to serialize Maps 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.

[JacksonJang]

I just submitted the PR.
I was considering branching based on the shape (POJO or OBJECT), but ended up using empty instead.
I wasn’t completely sure which approach would be better, so I pushed it.