Ensure the right values are set in SeriesGroupBy.nunique
We only need to use the group boundaries as the index for `res` so that
the dimensions match those of `out`. Fixes #13453

 - [x] closes #13453
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

can you add all 4 comparisions (e.g. ``.count`` == ``.nunique``, https://github.com/pandas-dev/pandas/issues/13453#issue-160499155) and using ``.agg`` https://github.com/pandas-dev/pandas/issues/13453#issuecomment-226295096

so you have 4 cases that you are comparing for equality (you can call them result1-4 and compare) pls also add a bug fix note in 0.20.0. lgtm otherwise. ping on green! (with test additions) @aiguofer you shouldn't have *any* tests fail. Note we have recently switched to ``pytest``. Added test to `resample` since it made more sense there... I also added `.agg('nunique')` for completeness.

When I run tests with `pytest` all passes :) # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15418?src=pr&el=h1) Report
> Merging [#15418](https://codecov.io/gh/pandas-dev/pandas/pull/15418?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d6f8b460325fd79faa90858e2743878a7cc74dec?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.


```diff
@@           Coverage Diff           @@
##           master   #15418   +/-   ##
=======================================
  Coverage   90.36%   90.36%           
=======================================
  Files         135      135           
  Lines       49438    49438           
=======================================
  Hits        44675    44675           
  Misses       4763     4763
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15418?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/compare/d6f8b460325fd79faa90858e2743878a7cc74dec...c53bd70743703173640766defd518bcee8dfaecd?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `94.89% <100%> (ø)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15418?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/15418?src=pr&el=footer). Last update [d6f8b46...c53bd70](https://codecov.io/gh/pandas-dev/pandas/compare/d6f8b460325fd79faa90858e2743878a7cc74dec...c53bd70743703173640766defd518bcee8dfaecd?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). @jreback all looks good :)
 thanks @aiguofer 

keep em coming!