API: joining DatetimeIndexes not preserving freq
```
dti = pd.date_range('2016-01-01', periods=10)
left, right = dti[:5], dti[5:]
joined = left.join(right, how="outer")
```

In master `joined.freq` comes back as None, while in 0.25.3 it was "D".  There is some code in DatetimeIndex._wrap_joined_index that suggests to me this was intentional.

Can someone confirm this suspicion cc @jorisvandenbossche ?  If so, I've got a branch that restores the old behavior, just need to add tests.