Fix issue when different class is defined in attrs for th and td

[ianalis]

When class is defined differently for th and td in the attrs attribute, for example,

class TestTable(ColumnShiftTableBootstrap5):
    test_col = tables.Column(
        attrs={"th": {"class": "text-nowrap"}, "td": {"class": "text-center"}}
    )

column hiding doesn't work because the value for data-td-class becomes wrong. This PR fixes this by setting it to the column name itself which is always a class of th and td.