BUG: case argument ignored when regex is False in Series.str.replace

[simonjayhawkins]

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• 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 detailing how to provide the necessary information for us to reproduce your bug.

Code Sample, a copy-pastable example

>>> 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()

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