add test case when to_csv argument is sys.stdout
- [+] closes #21561
- [+] tests added / passed
- [+] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Add new test case when to_csv argument is sys.stdout
Hello @r00ta! Thanks for updating the PR.

 - In the file [`pandas/tests/io/formats/test_to_csv.py`](https://github.com/pandas-dev/pandas/blob/557fa9fd252db5c4aeb1bcfbf73ed6da4917decb/pandas/tests/io/formats/test_to_csv.py), following are the PEP8 issues :

> [Line 291:60](https://github.com/pandas-dev/pandas/blob/557fa9fd252db5c4aeb1bcfbf73ed6da4917decb/pandas/tests/io/formats/test_to_csv.py#L291): [W291](https://duckduckgo.com/?q=pep8%20W291) trailing whitespace



##### Comment last updated on June 22, 2018 at 13:01 Hours UTC Is it possible to parametrize this as a value in an existing test rather than adding a separate test case? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21572?src=pr&el=h1) Report
> Merging [#21572](https://codecov.io/gh/pandas-dev/pandas/pull/21572?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/1638331172cb260c7c642d02e5cf97373f7bcba6?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21572/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/21572?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #21572      +/-   ##
==========================================
+ Coverage    91.9%    91.9%   +<.01%     
==========================================
  Files         153      153              
  Lines       49557    49549       -8     
==========================================
- Hits        45545    45539       -6     
+ Misses       4012     4010       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.78% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21572?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21572/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.27% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/21572/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.22% <0%> (ø)` | :arrow_up: |
| [pandas/core/tools/numeric.py](https://codecov.io/gh/pandas-dev/pandas/pull/21572/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvbnVtZXJpYy5weQ==) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/core/indexing.py](https://codecov.io/gh/pandas-dev/pandas/pull/21572/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhpbmcucHk=) | `93.37% <0%> (+0.13%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21572?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/21572?src=pr&el=footer). Last update [1638331...b250259](https://codecov.io/gh/pandas-dev/pandas/pull/21572?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks for the tips! I've updated the PR :D  @r00ta : You have a couple of linting errors:

https://travis-ci.org/pandas-dev/pandas/jobs/395098214#L2876 lgtm. merge on green. The remaining failure is a linting error:

pandas/tests/io/formats/test_to_csv.py:10:1: E302 expected 2 blank lines, found 1 
pandas/tests/io/formats/test_to_csv.py:291:60: W291 trailing whitespace thanks @r00ta 