## What changes were proposed in this pull request?
When Arrow optimization is enabled in Python 2.7,
```python
import pandas
pdf = pandas.DataFrame(["test1", "test2"])
df = spark.createDataFrame(pdf)
df.show()
```
I got the following output:
```
+----------------+
| 0|
+----------------+
|[74 65 73 74 31]|
|[74 65 73 74 32]|
+----------------+
```
This looks because Python's `str` and `byte` are same. it does look right:
```python
>>> str == bytes
True
>>> isinstance("a", bytes)
True
```
To cut it short:
1. Python 2 treats `str` as `bytes`.
2. PySpark added some special codes and hacks to recognizes `str` as string types.
3. PyArrow / Pandas followed Python 2 difference
To fix, we have two options:
1. Fix it to match the behaviour to PySpark's
2. Note the differences
but Python 2 is deprecated anyway. I think it's better to just note it and for go option 2.
## How was this patch tested?
Manually tested.
Doc was checked too:
Closes#24838 from HyukjinKwon/SPARK-27995.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
1217996f15
