BUG: resample with TimedeltaIndex, fenceposts are off
- [x] closes #13022
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Could you also add a downsampling test case as noted here:

https://github.com/pandas-dev/pandas/issues/13022#issuecomment-238382933 @mroeschke 

There is test `TestTimedeltaIndex.test_asfreq` that exactly reproduces mentioned comment and it's passing.  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22488?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@a5fe9cf`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22488/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22488?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22488   +/-   ##
=========================================
  Coverage          ?   92.04%           
=========================================
  Files             ?      169           
  Lines             ?    50782           
  Branches          ?        0           
=========================================
  Hits              ?    46741           
  Misses            ?     4041           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.45% <ø> (?)` | |
| #single | `42.3% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22488?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/resample.py](https://codecov.io/gh/pandas-dev/pandas/pull/22488/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzYW1wbGUucHk=) | `96.27% <ø> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22488?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/22488?src=pr&el=footer). Last update [a5fe9cf...bd8f31a](https://codecov.io/gh/pandas-dev/pandas/pull/22488?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Travis flagged a linting error: `pandas/tests/test_resample.py:29:1: F401 'pandas.tseries.frequencies.to_offset' imported but unused` @jreback  Hello @discort! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/resample.py`](https://github.com/discort/pandas/blob/bd8f31a92b1e03d1fd6141fd837969caafdf1a8f/pandas/core/resample.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_resample.py`](https://github.com/discort/pandas/blob/bd8f31a92b1e03d1fd6141fd837969caafdf1a8f/pandas/tests/test_resample.py) !

 thanks @discort 