[SPARK-23334][SQL][PYTHON] Fix pandas_udf with return type StringType() to handle str type properly in Python 2.

## What changes were proposed in this pull request?

In Python 2, when `pandas_udf` tries to return string type value created in the udf with `".."`, the execution fails. E.g.,

```python
from pyspark.sql.functions import pandas_udf, col
import pandas as pd

df = spark.range(10)
str_f = pandas_udf(lambda x: pd.Series(["%s" % i for i in x]), "string")
df.select(str_f(col('id'))).show()
```

raises the following exception:

```
...

java.lang.AssertionError: assertion failed: Invalid schema from pandas_udf: expected StringType, got BinaryType
	at scala.Predef$.assert(Predef.scala:170)
	at org.apache.spark.sql.execution.python.ArrowEvalPythonExec$$anon$2.<init>(ArrowEvalPythonExec.scala:93)

...
```

Seems like pyarrow ignores `type` parameter for `pa.Array.from_pandas()` and consider it as binary type when the type is string type and the string values are `str` instead of `unicode` in Python 2.

This pr adds a workaround for the case.

## How was this patch tested?

Added a test and existing tests.

Author: Takuya UESHIN <ueshin@databricks.com>

Closes #20507 from ueshin/issues/SPARK-23334.