[summary] FIX: Ensure index name is retained (#424)

[summary] FIX: Ensure index name `DATE` is retained when using `TIMESTEP` to improve index `DATE` index resolution

ecl2df/summary.py

@@ -483,7 +483,7 @@ def _ensure_unique_datetime_index(dframe: pd.DataFrame) -> pd.DataFrame:
                     "Dataframe of smry data contained duplicate timestamps",
                     "Vector TIMESTEP exists, but unit could not be identified",
                 )
-            dframe.index = index_as_list
+            dframe.index = pd.Series(data=index_as_list, name=dframe.index.name)
         else:
             raise ValueError(
                 "Dataframe of smry data contained duplicate timestamps due to limited.",

tests/test_summary.py

@@ -795,6 +795,18 @@ def test_unique_datetime_for_short_timesteps(filepath):
     assert summary.df(EclFiles(filepath)).index.is_unique
 
 
+@pytest.mark.parametrize(
+    "filepath",
+    [
+        SHORT_STEP_WITH_TIMESTEP,
+        SHORT_STEP_WITH_TIMESTEP_LONG,
+    ],
+)
+def test_unique_datetime_retain_index_name(filepath):
+    """Test _ensure_unique_datetime_index method retain index name"""
+    assert summary.df(EclFiles(filepath)).index.name == "DATE"
+
+
 def test_smry_meta():
     """Test obtaining metadata dictionary for summary vectors from an EclSum object"""
     meta = smry_meta(EclFiles(REEK))