BUG: Fix default encoding for CSVFormatter.save
- [x] closes #17097
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=h1) Report
> Merging [#17821](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9f0ee53dad3bd14a3f0475a29d326c89c6b6aced?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `87.5%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17821/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17821      +/-   ##
==========================================
- Coverage   91.26%   91.24%   -0.03%     
==========================================
  Files         163      163              
  Lines       49980    49985       +5     
==========================================
- Hits        45613    45607       -6     
- Misses       4367     4378      +11
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <87.5%> (-0.01%)` | :arrow_down: |
| #single | `40.24% <62.5%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.94% <87.5%> (-0.13%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.74% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17821?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/17821?src=pr&el=footer). Last update [9f0ee53...8263456](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=h1) Report
> Merging [#17821](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9f0ee53dad3bd14a3f0475a29d326c89c6b6aced?src=pr&el=desc) will **decrease** coverage by `0.05%`.
> The diff coverage is `87.5%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17821/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17821      +/-   ##
==========================================
- Coverage   91.26%    91.2%   -0.06%     
==========================================
  Files         163      163              
  Lines       49980    50019      +39     
==========================================
+ Hits        45613    45621       +8     
- Misses       4367     4398      +31
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.01% <87.5%> (-0.04%)` | :arrow_down: |
| #single | `40.24% <62.5%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.94% <87.5%> (-0.13%)` | :arrow_down: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb21wYXQucHk=) | `62% <0%> (-6.19%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `63.38% <0%> (-1.82%)` | :arrow_down: |
| [pandas/core/nanops.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbmFub3BzLnB5) | `96.67% <0%> (-0.99%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.77% <0%> (-0.08%)` | :arrow_down: |
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `96.94% <0%> (ø)` | :arrow_up: |
| [pandas/core/sparse/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3BhcnNlL3Nlcmllcy5weQ==) | `95.26% <0%> (ø)` | :arrow_up: |
| [pandas/tests/indexes/conftest.py](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree#diff-cGFuZGFzL3Rlc3RzL2luZGV4ZXMvY29uZnRlc3QucHk=) | | |
| ... and [5 more](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17821?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/17821?src=pr&el=footer). Last update [9f0ee53...c222ee8](https://codecov.io/gh/pandas-dev/pandas/pull/17821?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @gfyoung Thanks for your review. Fixed! @Licht-T this looks good, can you add a whatsnew note (bug fixes section is ok).

also pls look and see if this change may close any/all of (if it does just indicate and include that issue in the note as well): #13068, #10813, #9712, #10755 AFAICT, none of those issues were closed by this one. Sorry, I also added the whatsnew note @jreback @TomAugspurger This PR does not solve these issue, I think. The CI failure here was from the matplotlib 2.1 release. It'll be fixed when the merge commit is tested.

Thanks @Licht-T 