BUG: loffset not applied when using resample with agg() (GH13218)
- [x] closes #13218  and #15002 (continuation of closed PR https://github.com/pydata/pandas/pull/13861)
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

_apply_loffset is called in resampler's downsample and groupby_and_agg methods, defined [here](https://github.com/pydata/pandas/blob/master/pandas/tseries/resample.py#L535) and [here](https://github.com/pydata/pandas/blob/master/pandas/tseries/resample.py#L545)

When a `str` is passed into `aggregate`, the above methods are called [here](https://github.com/pydata/pandas/blob/master/pandas/core/base.py#L431), and consequently the loffset is applied. 

**However**, if anything other than a `str` is passed into `aggregate`, then a different path will be taken and **the loffset must be applied**. So a simple check in `aggregate` will apply the loffset correctly.

note: my last PR was closed, and I can't reopen it by a force push, hence the new one

## [Current coverage](https://codecov.io/gh/pandas-dev/pandas/pull/14213?src=pr) is 84.78% (diff: 100%)
> Merging [#14213](https://codecov.io/gh/pandas-dev/pandas/pull/14213?src=pr) into [master](https://codecov.io/gh/pandas-dev/pandas/branch/master?src=pr) will increase coverage by **<.01%**

```diff
@@             master     #14213   diff @@
==========================================
  Files           145        145          
  Lines         51090      51094     +4   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43315      43321     +6   
+ Misses         7775       7773     -2   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [e27b296...8619e9e](https://codecov.io/gh/pandas-dev/pandas/compare/e27b29697f0dcf9359f01a19edb2f20c6d728b6c...8619e9efa1ddd32397bab44cdea959b8f9ddc221?src=pr) @wcwagner if you want to update to 0.19.2 and rebase, we can take another look. can you move release note to 0.20.0 Sorry for the delay, I will update everything tomorrow. pls add example referenced in #15002 as well (your changes should cover it) @jreback do you want me to add the example in #15002 into the unittests? I suppose it would be a good idea to do so, seeing as it tests the `how` parameter (even though it's deprecated). yep Seems like resample's loffset tests were removed from `TestPeriodIndex` and `TestTimedeltaIndex`. Should I remove my tests from those two as well? thanks @wcwagner 

I reworked this a bit to make it cleaner in https://github.com/pandas-dev/pandas/commit/366241386282272328333256b6e267a68f8133a4

love for you to tackle some more issues!