Fields extension: specifying both `exclude` and `include` for the same field does not result in expected behavior

[mcucchi9]

Tag: 6.2.1

The STAC API specification for the Fields extension states that (point 6 here):

If the same field is present in both include and exclude, it should be included.

On the contrary, performing a POST /search request to an instance of the stac-fastapi-pgstac API with the following payload

{
    "fields": {
        "exclude": ["properties.datetime"],
        "include": ["properties.datetime"]
    }
}

results in a FeatureCollection in which each Feature only have the fields id, collection and links.

Digging into the code, I can see this behavior is probably due to the logic in the filter_fields function of stac_fastapi/pgstac/utils.py, in which the exclude and include operations are applied sequentially and in this order.