BUG: Index string ops ignores the order

#### Series (OK)

```
s = pd.Series(['A', 'B'])
'x' + s
#0    xA
#1    xB
# dtype: object

s + 'y'
#0    Ay
#1    By
# dtype: object
```
#### Index (NG)

```
idx = pd.Index(['A', 'B'])

# NG
'x' + idx
# Index([u'Ax', u'Bx'], dtype='object')

# OK
idx + 'y'
# Index([u'Ay', u'By'], dtype='object')
```
