BUG: XlsxWriter ignoring formats on numpy types if merged cells
The XlsxWriter `write_cells` method takes a `cells` parameter that is itself derived from a generator of the various cells in the DataFrame. It calls the `_value_with_format()` to convert any numpy types to Python types for the Excel writer. 

In the section of code that deals with writing merged cells, the original passed `cell.val` parameter was being passed into the writer, rather than the `val` returned from the `_value_with_format()` function. This
caused incompatible numpy or Pandas types to get passed into the writer when the condition of being in a 'merged' (grouped) DataFrame cell was met. 

In my case, I had a Pandas `Period` object in the MultiIndex of a grouped DataFrame . All that needed to be done to fix this was simply removing the `cell.` from `cell.val` that was being passed into `wks.merge_range()` function, and replacing it with the correct `val`, which is 'type safe' for the Excel writer.

- [x] closes #26999 
- [x] tests added / passed
- [x] whatsnew entry
