TableOne with categorical pandas DataFrame column raises TypeError

[tompollard]

As highlighted in #177, setting include_null = True will fail for categorical data types (i.e. if a column has been converted to the Pandas Categorical type, include_null=True will raise an error).

After applying this fix, the following code should add the "None" category as expected:

import tableone
import pandas as pd

dummy_table = pd.DataFrame(
    {
        "age": [70, 80, 90, 85, 99],
        "sex": ["m", "f", "m", "f", None]
    }
)
dummy_table["sex"] = dummy_table["sex"].astype("category")

tableone.TableOne(dummy_table)

Returns:

		Missing	Overall
n			5
age, mean (SD)		0	84.8 (10.8)
sex, n (%)	None		1 (20.0)
	f		2 (40.0)
	m		2 (40.0)