ENH: formatting integers in FloatIndex as floats
Ref issue #12164

Previous Behavior:

``` python
   In [2]: s = Series([1,2,3], index=np.arange(3.))

   In [3]: s
   Out[3]:
   0    1
   1    2
   2    3
   dtype: int64

   In [4]: s.index
   Out[4]: Float64Index([0.0, 1.0, 2.0], dtype='float64')

   In [5]: print(s.to_csv(path=None))
   0,1
   1,2
   2,3
```

New behavior

``` python
   In [2]: s = Series([1,2,3], index=np.arange(3.))

   In [3]: s
   Out[3]:
   0.0    1
   1.0    2
   2.0    3
   dtype: int64


   In [4]: s.index
   Out[4]: Float64Index([0.0, 1.0, 2.0], dtype='float64')

   In [5]: print(s.to_csv(path=None))
   0.0,1
   1.0,2
   2.0,3
```
