preserve boolean-dtype within encoding, adapt test

Author: kmuehlbauer

xarray/conventions.py

@@ -400,7 +400,7 @@ def decode_cf_variable(
     encoding.setdefault("dtype", original_dtype)
 
     if "dtype" in attributes and attributes["dtype"] == "bool":
-        del attributes["dtype"]
+        encoding["dtype"] = attributes.pop("dtype")
         data = BoolTypeArray(data)
 
     if not is_duck_dask_array(data):

xarray/tests/test_backends.py

@@ -632,6 +632,7 @@ def test_roundtrip_boolean_dtype(self) -> None:
         with self.roundtrip(original) as actual:
             assert_identical(original, actual)
             assert actual["x"].dtype == "bool"
+            assert actual["x"].encoding["dtype"] == "bool"
 
     def test_orthogonal_indexing(self) -> None:
         in_memory = create_test_data()