Save a formula or any string which starts with `"="` as a string using the cells-based interface

[miron1512]

Hi @theoephraim, There is currently no way to save a formula or any string which starts with "=" as a string using the cells-based interface. I think that if we explicitly use cell.stringValue, we already know that we want to save the value as string, regardless of whether it's a formula or any string which starts with "=". So, we don't need to throw an error here:

I created a small PR to fix this: #643