API: DataFrame.to_csv formatting parameters for float indexes
Fix issue #11553 

Two things:

1) I've created a `Float64Index._format_native_types` method which is a copy-paste of  `FloatBlock.to_native_types`. I would have preferred to call the latter directly, but I'm not sure what the `placement` parameter of the `FloatBlock` constructor means. I guess I doesn't really matters, since I could put whatever value and it should work (I think), and my hesitation a bit unfounded, but I don't know if it would be really a clean solution. Maybe someone can think of a more elegant way?

2) Since a `Float64Index` containing only NaNs collapses when part of a multi-index, its NaNs values would not be converted using `na_rep`, so I had to hack a solution. I put a comment in the relevant part. I'm not quite convinced myself of the elegance of the solution, though.

What do you think?
