BUG/API: allow get_indexer to work with nans
- [ ] clear the FIXME in Categorical in #7820 (comment)

closes #7820

looks ok conceptually...but failing tests.
 yeah a few kinks to work out 
 @cpcloud can you circle back to this when you have a chacne?
 sure
 If this is fixed, the workaround in categorical.py should be removed:

```
        # FIXME: the following can be removed after https://github.com/pydata/pandas/issues/7820
        # is fixed.
        # float levels do currently return -1 for np.nan, even if np.nan is included in the index
        # "repair" this here
        if com.isnull(rvalue).any() and com.isnull(self.levels).any():
            nan_pos = np.where(com.isnull(self.levels))
            lindexer[lindexer == -1] = nan_pos
```

[This is in #8007, so not yet merged]
 @cpcloud ?
 @cpcloud ?
 @cpcloud status?
 @cpcloud can you update?
 closing
