BUG: case argument ignored when regex is False in Series.str.replace
- [ ] I have checked that this issue has not already been reported.

- [ ] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

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**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

#### Code Sample, a copy-pastable example

```python
>>> ser = pd.Series(["A.", "a.", "Ab", "ab", np.nan], dtype="object")
>>> ser
0     A.
1     a.
2     Ab
3     ab
4    NaN
dtype: object
>>> 
>>> ser.str.replace("a", "c", case=False, regex=False)
0     A.
1     c.
2     Ab
3     cb
4    NaN
dtype: object
>>> 
```

#### Problem description

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution]

#### Expected Output

```
>>> ser.str.replace("a", "c", case=False, regex=False)
0     c.
1     c.
2     cb
3     cb
4    NaN
dtype: object
>>> 
```
#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here leaving a blank line after the details tag]

</details>
