Make is_monotonic~ work properly for index (#930)

Resolve #931 (The contents below is same as the contents of this issue)

When we use `is_monotonic_decreasing` or `is_monotonic_increasing` for index,

pandas works like below:

```python
>>> s = pd.Series([7, 6, 5, 4, 3, 2, 1], index=[7, 6, 5, 4, 3, 2, 1])
>>> s.index.is_monotonic_decreasing
True
```
Since the index order is literally motononic decreasing, they return `True`

But our case,

```python
>>> s = ks.Series([7, 6, 5, 4, 3, 2, 1], index=[7, 6, 5, 4, 3, 2, 1])
>>> s.index.is_monotonic_decreasing
False
```

as seen above, it returns `False`.

because our existing logic always order by index column before calculate the each rows increasing or decreasing.

So they always calculate result based on data column, not index column. (index column is always sorted since order by)

So i think maybe it is better to fix this logic to follow behavior of pandas one for index.

Author: itholic

databricks/koalas/base.py

@@ -27,6 +27,7 @@
 from pyspark import sql as spark
 from pyspark.sql import functions as F, Window
 from pyspark.sql.types import DoubleType, FloatType, LongType, StringType, TimestampType
+from pyspark.sql.functions import monotonically_increasing_id
 
 from databricks import koalas as ks  # For running doctests and reference resolution in PyCharm.
 from databricks.koalas.internal import _InternalFrame
@@ -299,9 +300,16 @@ def is_monotonic(self):
 
         >>> ser.rename("a").to_frame().set_index("a").index.is_monotonic
         True
+
+        >>> ser = ks.Series([5, 4, 3, 2, 1], index=[1, 2, 3, 4, 5])
+        >>> ser.is_monotonic
+        False
+
+        >>> ser.index.is_monotonic
+        True
         """
         col = self._scol
-        window = Window.orderBy(self._kdf._internal.index_scols).rowsBetween(-1, -1)
+        window = Window.orderBy(monotonically_increasing_id()).rowsBetween(-1, -1)
         return self._with_new_scol((col >= F.lag(col, 1).over(window)) & col.isNotNull()).all()
 
     is_monotonic_increasing = is_monotonic
@@ -343,9 +351,16 @@ def is_monotonic_decreasing(self):
 
         >>> ser.rename("a").to_frame().set_index("a").index.is_monotonic_decreasing
         True
+
+        >>> ser = ks.Series([5, 4, 3, 2, 1], index=[1, 2, 3, 4, 5])
+        >>> ser.is_monotonic_decreasing
+        True
+
+        >>> ser.index.is_monotonic_decreasing
+        False
         """
         col = self._scol
-        window = Window.orderBy(self._kdf._internal.index_scols).rowsBetween(-1, -1)
+        window = Window.orderBy(monotonically_increasing_id()).rowsBetween(-1, -1)
         return self._with_new_scol((col <= F.lag(col, 1).over(window)) & col.isNotNull()).all()
 
     def astype(self, dtype):