BUG: support "fill_value" for ".unstack()" called with list of levels
- [x] closes #13971
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Notice this is a simple fix which doesn't make @kordek 's [more general comment](https://github.com/pandas-dev/pandas/issues/13971#issuecomment-243558072) about the opportunity for refactoring less valid.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=h1) Report
> Merging [#17887](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/00f23ca69e8999592f1bad02c6d763df48365a97?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17887/graphs/tree.svg?height=150&token=eZ4WkYLtcO&width=650&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17887      +/-   ##
==========================================
- Coverage   91.23%   91.21%   -0.02%     
==========================================
  Files         163      163              
  Lines       50102    50102              
==========================================
- Hits        45712    45703       -9     
- Misses       4390     4399       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.03% <100%> (ø)` | :arrow_up: |
| #single | `40.31% <25%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.27% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17887?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/17887?src=pr&el=footer). Last update [00f23ca...2d56672](https://codecov.io/gh/pandas-dev/pandas/pull/17887?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/17887?src=pr&el=h1) Report
> Merging [#17887](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9092445b1b8bdc6cb031a2319ac98204b13f908c?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/17887/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17887      +/-   ##
==========================================
+ Coverage   91.23%   91.24%   +<.01%     
==========================================
  Files         163      163              
  Lines       50105    50105              
==========================================
+ Hits        45714    45718       +4     
+ Misses       4391     4387       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.05% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.31% <25%> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `99.27% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.5% <0%> (+0.09%)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.2% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17887?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/17887?src=pr&el=footer). Last update [9092445...74a2104](https://codecov.io/gh/pandas-dev/pandas/pull/17887?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm minor comments. thanks @toobaz 