to_datetime loses freq on DatetimeIndex with offset
It would be cool if the offset were kept. Perhaps even an error should be raised if not all of the `Timestamp`s have the same `offset`:

```
In [36]:
dti = pd.to_datetime([pd.Timestamp("2014-1-1", offset="M"),])
dti

Out[36]:
<class 'pandas.tseries.index.DatetimeIndex'>
[2014-01-01]
Length: 1, Freq: None, Timezone: None
```

See #6560 
