BUG: Categorical constructor not idempotent with ext dtype

closes #14190

Author: Chris <cbartak@gmail.com>

Closes #14191 from chris-b1/cat-ctor and squashes the following commits:

4cad147 [Chris] add some nulls to tests
da865e2 [Chris] BUG: Categorical constructor not idempotent with ext dtype

doc/source/whatsnew/v0.19.0.txt

@@ -1426,7 +1426,7 @@ Bug Fixes
 
 - Bug in ``SeriesGroupBy.transform`` with datetime values and missing groups (:issue:`13191`)
 - Bug where empty ``Series`` were incorrectly coerced in datetime-like numeric operations (:issue:`13844`)
-
+- Bug in ``Categorical`` constructor when passed a ``Categorical`` containing datetimes with timezones (:issue:`14190`)
 - Bug in ``Series.str.extractall()`` with ``str`` index raises ``ValueError``  (:issue:`13156`)
 - Bug in ``Series.str.extractall()`` with single group and quantifier  (:issue:`13382`)
 - Bug in ``DatetimeIndex`` and ``Period`` subtraction raises ``ValueError`` or ``AttributeError`` rather than ``TypeError`` (:issue:`13078`)

pandas/core/categorical.py

@@ -259,7 +259,7 @@ def __init__(self, values, categories=None, ordered=False,
                 ordered = values.ordered
             if categories is None:
                 categories = values.categories
-            values = values.__array__()
+            values = values.get_values()
 
         elif isinstance(values, (ABCIndexClass, ABCSeries)):
             pass

pandas/tests/test_categorical.py

@@ -362,6 +362,22 @@ def test_constructor_from_index_series_period(self):
         result = pd.Categorical(pd.Series(idx))
         tm.assert_index_equal(result.categories, idx)
 
+    def test_constructor_invariant(self):
+        # GH 14190
+        vals = [
+            np.array([1., 1.2, 1.8, np.nan]),
+            np.array([1, 2, 3], dtype='int64'),
+            ['a', 'b', 'c', np.nan],
+            [pd.Period('2014-01'), pd.Period('2014-02'), pd.NaT],
+            [pd.Timestamp('2014-01-01'), pd.Timestamp('2014-01-02'), pd.NaT],
+            [pd.Timestamp('2014-01-01', tz='US/Eastern'),
+             pd.Timestamp('2014-01-02', tz='US/Eastern'), pd.NaT],
+        ]
+        for val in vals:
+            c = Categorical(val)
+            c2 = Categorical(c)
+            tm.assert_categorical_equal(c, c2)
+
     def test_from_codes(self):
 
         # too few categories