Series.sort_values using mergesort and descending is not stable
#### Code Sample

```python
import pandas as pd

s = pd.Series([1, 2, 1, 3], ['first', 'b', 'second', 'c'])
print(s.sort_values(ascending=False, kind='mergesort'))
```
####  Output
```
c         3
b         2
second    1
first     1
dtype: int64
```

####  Expected Output
```
c         3
b         2
first     1
second    1
dtype: int64
```
#### Problem description
Mergesort should be stable irrespective of direction, but `ascending=False` (which is not stable) is just the reverse of `ascending=True` (which is stable). 