[BUG] datetime column (pd.DataFrame) returned in Transformer is causing spark error

[goodwanghan]

Minimal Code To Reproduce

import pandas as pd

# schema: a:datetime
def t(sdf:pd.DataFrame) -> pd.DataFrame:
    sdf["a"]=pd.to_datetime(sdf["a"])
    return sdf

with FugueSQLWorkflow(SparkExecutionEngine()) as dag:
    dag.df([["2020-01-01"]], "a:str").transform(t).show()

Describe the bug

TypeError: field a: TimestampType can not accept object Timestamp('2020-01-01 00:00:00') in type <class 'pandas._libs.tslibs.timestamps.Timestamp'>

Expected behavior
This should work.
Should add datetime tests into general execution engine test suites.

Environment (please complete the following information):

• Backend: pandas/dask/ray? spark
• Backend version: 3
• Python version: 3.6.9
• OS: linux/windows linux

[goodwanghan]

This problem seems only affects pyarrow<0.15.1