using DataFrame.resample with 'agg' method on non-existant columns provides unexpected behavior
- [x] closes #16766
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Looks like your change is causing failures elsewhere. I'm not personally familiar with this code, I'm not sure what the best way forward is. Let me know if you need help though, and I can take a look. Hello @discort! Thanks for updating the PR.





Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on February 07, 2018 at 08:24 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=h1) Report
> Merging [#19552](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/983d71fa8477439aaa227367d7f2f14952e4e235?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19552/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19552      +/-   ##
==========================================
+ Coverage    91.6%    91.6%   +<.01%     
==========================================
  Files         150      150              
  Lines       48750    48752       +2     
==========================================
+ Hits        44656    44660       +4     
+ Misses       4094     4092       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.97% <100%> (ø)` | :arrow_up: |
| #single | `41.75% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19552/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYmFzZS5weQ==) | `96.78% <100%> (+0.01%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19552/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.85% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=footer). Last update [983d71f...d142bc1](https://codecov.io/gh/pandas-dev/pandas/pull/19552?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger  thanks! keep em coming!