DOC: Update is_sparse docstring
`is_sparse(pd.SparseDataFrame([1,2,3]))` returns `False`.

The is_sparse function checks to see if
an array-like is spare by checking to see
if it is an instance of ABCSparseArray or
ABCSparseSeries. This commit adds
ABCSparseDataFrame to that list -- so it
can detect that a DataFrame (which is an
array-like object) is sparse.

Added a test for this.

- [ ] closes #xxxx
- [ ] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

why is this necessary? ``is_sparse`` is for checking 1-d, a DataFrame is 2-d. I was looking at this method for the documentation sprint, and I thought it was odd that it would return False for a for `SparseDataFrame`. I wasn't too sure if array-like just referred to 1-d arrays or 2-d as well. A note could be added that it expects a 1-d array - and that would resolve it as well. a doc-string update would be great. Hello @sechilds! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/dtypes/common.py`](https://github.com/sechilds/pandas/blob/227007a868468153b85d0a28742bcb694c6b9bef/pandas/core/dtypes/common.py) !



##### Comment last updated on November 03, 2018 at 05:36 Hours UTC @datapythonista I think this is ready to be merged.  @sechilds can you rebase? Sorry your PR was forgotten. No problem. I will rebase it later today. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=h1) Report
> Merging [#19983](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/20bdb3e4019afe2037b26524a447ac8913d6e325?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19983/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19983   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51317    51317           
=======================================
  Hits        47338    47338           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.37% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19983?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/19983?src=pr&el=footer). Last update [20bdb3e...5a78a39](https://codecov.io/gh/pandas-dev/pandas/pull/19983?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/19983?src=pr&el=h1) Report
> Merging [#19983](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/4f717551b8e49ecd978bb4f5a985d6f6a96d86e9?src=pr&el=desc) will **decrease** coverage by `0.5%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19983/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19983      +/-   ##
==========================================
- Coverage   92.22%   91.71%   -0.51%     
==========================================
  Files         161      150      -11     
  Lines       51187    49112    -2075     
==========================================
- Hits        47209    45045    -2164     
- Misses       3978     4067      +89
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.1% <ø> (-0.52%)` | :arrow_down: |
| #single | `41.86% <ø> (-0.4%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.64% <ø> (+0.27%)` | :arrow_up: |
| [pandas/plotting/\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb21wYXQucHk=) | `62% <0%> (-25.5%)` | :arrow_down: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `74.35% <0%> (-23.66%)` | :arrow_down: |
| [pandas/io/s3.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3MzLnB5) | `72.72% <0%> (-13.64%)` | :arrow_down: |
| [pandas/util/\_doctools.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RvY3Rvb2xzLnB5) | `0% <0%> (-12.88%)` | :arrow_down: |
| [pandas/core/reshape/util.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS91dGlsLnB5) | `90.32% <0%> (-9.68%)` | :arrow_down: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `82.4% <0%> (-8.95%)` | :arrow_down: |
| [pandas/core/dtypes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Jhc2UucHk=) | `91.66% <0%> (-8.34%)` | :arrow_down: |
| [pandas/errors/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2Vycm9ycy9fX2luaXRfXy5weQ==) | `92.3% <0%> (-7.7%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `92.04% <0%> (-6.38%)` | :arrow_down: |
| ... and [142 more](https://codecov.io/gh/pandas-dev/pandas/pull/19983/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19983?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/19983?src=pr&el=footer). Last update [4f71755...227007a](https://codecov.io/gh/pandas-dev/pandas/pull/19983?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rebased, merging on green. Pushed one more rebase and merging on green. Thanks @sechilds and @datapythonista 