DataFrame combine_first() loses timezone information for datetime columns
xref addl example in #13650 

combine_first() loses timezone information for datetime columns

```
dts1 = pd.date_range('20150101','20150105',tz='UTC')
df1 = pd.DataFrame({'DATE':dts1})
dts2 = pd.date_range('20150103','20150105',tz='UTC')
df2 = pd.DataFrame({'DATE':dts2})
df = df1.combine_first(df2)
df.DATE[0].tz # this shows up as None
```
