Allow non-numeric/date value into number/date formatted cell

[hnx8]

1. Partly related to #189 : When specifying fraction '# ??? / ???' by format specification, it is not a source code implementation that determines cell type as 'n_numeric'. (Strictly, '#' '?' is not described to recognize it as a numeric type, but regex '/$/ absolutely determines as 'n_numeric')
2. When assign a non-numeric value to the number formatted cell (In case I want to set a fixed text message for some reason), opening it with Excel will result in an error stating that the file has been corrupted.
3. When assign a non-date value to the date/datetime formatted cell (In case I want to set a fixed text message for some reason), The value of the cell becomes "1900/01/01".

In order to avoid 2, 3, added processing to check the data type when setting the value in the cell.