fix geometry detection (#573)

Author: martinfleis

cf_xarray/geometry.py

@@ -298,8 +298,10 @@ def encode_geometries(ds: xr.Dataset):
     geom_var_names = [
         name
         for name, var in ds._variables.items()
-        if var.dtype == "O" and isinstance(var.data.flat[0], SHAPELY_TYPES)
+        if var.dtype == "geometry"
+        or (var.dtype == "O" and isinstance(var.data.flat[0], SHAPELY_TYPES))
     ]
+
     if not geom_var_names:
         return ds
 

cf_xarray/tests/test_geometry.py

@@ -507,5 +507,7 @@ def test_encode_decode(geometry_ds, polygon_geometry):
     )
     multi_ds = xr.merge([polyds, geometry_ds[1]])
     for ds in (geometry_ds[1], polygon_geometry.to_dataset(), geom_dim_ds, multi_ds):
-        roundtripped = decode_geometries(encode_geometries(ds))
+        encoded = encode_geometries(ds)
+        assert len(encoded.data_vars) > len(ds.data_vars)
+        roundtripped = decode_geometries(encoded)
         xr.testing.assert_identical(ds, roundtripped)