BUG: DatetimeTZBlock can't assign values near dst boundary
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Value assignment doesn't work if data contains DST boundary because of internal `.localize(None)`. 

```
s = pd.Series(pd.date_range('2016-11-06', freq='H', periods=3, tz='US/Eastern'))
s[1] = pd.Timestamp('2011-01-01', tz='US/Eastern')
s
#0   2016-11-06 00:00:00-04:00
#1   2016-11-06 01:00:00-04:00
#2   2016-11-06 01:00:00-05:00
# dtype: datetime64[ns, US/Eastern]
```
