GetFeature failure on clustered layer.

[m431m]

Following issue #5219 @szekerest.

LAYER
    NAME "foo"
    TYPE POINT
    CONNECTIONTYPE POSTGIS
    CONNECTION "..."
    PROCESSING "CLOSE_CONNECTION=DEFER"
    DATA "geom FROM table USING UNIQUE id USING srid=4326"
    METADATA
        "wfs_enable_request" "*"
        "gml_featureid" "gid"
        "gml_include_items" "all"
        # (...)
    END
    PROJECTION
        "init=epsg:4326"
    END
    CLUSTER
        MAXDISTANCE 50
        REGION "ellipse"
    END
END

A GetFeature request returns this warning below, while gml_featureid is set..

<!-- WARNING: FeatureId item 'gid' not found in typename 'foo' -->

So, the response document is empty.

Removes 'CLUSTER' and it normally works.