BUG: Right result for DatetimeIndex + TimeDelta when timezone is set(…
- [x] closes #13905 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

…#13905)

## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13935?src=pr) is 85.30% (diff: 100%)

> Merging [#13935](https://codecov.io/gh/pydata/pandas/pull/13935?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13935   diff @@
==========================================
  Files           139        139          
  Lines         50157      50161     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          42785      42790     +5   
+ Misses         7372       7371     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [b7abef4...b17da31](https://codecov.io/gh/pydata/pandas/compare/b7abef4949fb1ba7fd1004feba4f47ace7004282...b17da314e42ea0db4fbb60695e8c8f43334efb9c?src=pr)
 Pls add a test for `Index` constructor covering possible patterns.
 acutally need to localize if freq < 1D, otherwise can simply add it.

E.g. if we are crossing a dst boundary, then need to convert to UTC, operate, then re-convert.

So I think we should continue to always convert / operate / re-convert for simplicity.
 @jreback you're right, but `DatetimeIndex` itself handles this, so not needed in `ops.py`?
- https://github.com/pydata/pandas/blob/master/pandas/tseries/index.py#L771
 can you rebase / update?
 can you rebase / update
 can you rebase / update closing, but if you want to update, pls comment