BUG: set column when using fallback results

[ChengjieLi28]

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import xorbits.pandas as pd
xdf = pd.DataFrame([[1,2], [3,4]], columns=[["a", "b"], ["c", "d"]])
xdf.columns = xdf.columns.droplevel(0)
print(xdf)
print(xdf.columns)

The result is not correct.