BUG: Float64Index.get_value() for tuples.
- [x] closes #13509
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Currently, trying to retrieve one tuple-valued entry in a `Series` with
a `Float64Index` will fail (GH 13509). This fixes that bug.
