DataFrame.to_csv ignores some formatting parameters for float indexes
xref #11551 

Parameter `float_format` and `decimal` options are ignored in an Index, but work in the data itself.

```
In [2]: pd.DataFrame({'a': [0.1,1.1], 'b': [2, 3]}).to_csv(float_format='%.2f', index=False)
Out[2]: 'a,b\n0.10,2\n1.10,3\n

In [3]: pd.DataFrame({'a': [0.1,1.1], 'b': [2, 3]}).set_index('a').to_csv(float_format='%.2f')
Out[3]: 'a,b\n0.1,2\n1.1,3\n'
```

and

```
In [4]: pd.DataFrame({'a': [0.1,1.1], 'b': [2, 3]}).to_csv(decimal='^', index=False)
Out[4]: 'a,b\n0^1,2\n1^1,3\n'

In [4]: pd.DataFrame({'a': [0.1,1.1], 'b': [2, 3]}).set_index('a').to_csv(decimal='^')
Out[4]: 'a,b\n0.1,2\n1.1,3\n'
```

I'll do a PR, soon I hope :)
