Multi-index columns are not produced properly in the dataframe_to_rows function
There is a bug introduced when the `expand_index` function was generalised and added to dataframe_to_rows (https://foss.heptapod.net/openpyxl/openpyxl/-/blob/branch/3.0/openpyxl/utils/dataframe.py#L64) A Multi-index column of this shape: ``` col_name1 col_name2 col_name3 col_name1.1 col_name2.1 col_name2.2 col_name3.1 col_name3.2 ``` Is transformed into ``` col_name1 col_name1.1 col_name2.1 col_name2.2 col_name3.1 col_name3.2 ``` missing any of the subsequent column names in the first level FYI @charlie.clark
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