Fix and test TimedeltaIndex.__rfloordiv__ bug
I thought we were ready to get rid of _TimeOp, but doing so turned up a couple of new bugs.

Setup:
```
scalar_td = pd.Timedelta('10m7s')
tdi = TimedeltaIndex(['00:05:03', '00:05:03', pd.NaT], freq=None)
```

Before:
```
>>> tdi.__rfloordiv__(scalar_td)
Float64Index([0.0, 0.0, nan], dtype='float64')
```

After:
```
>>> tdi.__rfloordiv__(scalar_td)
Float64Index([2.0, 2.0, nan], dtype='float64')
```

- [ ] closes #xxxx
- [x] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
