Added multi-test and whatsnew note

Author: Robin

doc/source/whatsnew/v0.20.0.txt

@@ -924,3 +924,4 @@ Bug Fixes
 - Bug in ``pd.melt()`` where passing a tuple value for ``value_vars`` caused a ``TypeError`` (:issue:`15348`)
 - Bug in ``.eval()`` which caused multiline evals to fail with local variables not on the first line (:issue:`15342`)
 - Bug in ``pd.read_msgpack`` which did not allow to load dataframe with an index of type ``CategoricalIndex`` (:issue:`15487`)
+- Bug in ``.mode()`` where ``mode`` was not returned for a single value (:issue:`15714`)

pandas/tests/test_algos.py

@@ -1261,11 +1261,18 @@ def test_mode_single(self):
         exp_single = [1]
         data_single = [1]
 
+        exp_multi = [1]
+        data_multi = [1, 1]
+
         for dt in np.typecodes['AllInteger'] + np.typecodes['Float']:
             s = Series(data_single, dtype=dt)
             exp = Series(exp_single, dtype=dt)
             tm.assert_series_equal(algos.mode(s), exp)
 
+            s = Series(data_multi, dtype=dt)
+            exp = Series(exp_multi, dtype=dt)
+            tm.assert_series_equal(algos.mode(s), exp)
+
         exp = Series([1], dtype=np.int)
         tm.assert_series_equal(algos.mode([1]), exp)