BUG: fix Series constructor for scalar and Categorical dtype
Categories and ordering of the resulting Series were not the same as those of the passed Categorical dtype. Closes #19565

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

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19717?src=pr&el=h1) Report
> Merging [#19717](https://codecov.io/gh/pandas-dev/pandas/pull/19717?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/83820679f16e6803b653ede5a4e468a211fcb443?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19717/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/19717?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19717      +/-   ##
==========================================
- Coverage   91.61%    91.6%   -0.01%     
==========================================
  Files         150      150              
  Lines       48882    48882              
==========================================
- Hits        44782    44780       -2     
- Misses       4100     4102       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.98% <100%> (-0.01%)` | :arrow_down: |
| #single | `41.79% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19717?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/dtypes/cast.py](https://codecov.io/gh/pandas-dev/pandas/pull/19717/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2Nhc3QucHk=) | `87.98% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19717/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `83.64% <0%> (-0.21%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19717?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/19717?src=pr&el=footer). Last update [8382067...47f7d40](https://codecov.io/gh/pandas-dev/pandas/pull/19717?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Could you also add a test that hits the `Series` constructor directly?

```python
result = Series('a', index=[0, 1],
                dtype=pd.api.types.CategoricalDtype(['a', 'b'], ordered=True)
```

That way if the implementation changes in the future, we at least have that covered. thanks!