Researching a security advisory, the code for handling text colors in Html Writer when the cell is non-numeric and the color is hard-coded in the Number Format looked a bit suspicious. As it turns out, the code was *not* subject to a security exploit. However, the color for the Number Format was ignored. This PR corrects that omission.
This will improve round-tripping from Xlsx to Html and back again. The calculated result will still be what is displayed when viewing the result in a browser, but, using the new option, the original formula can be reconstructed when PhpSpreadsheet reads the Html file. The ability to recognize this situation is automatically enabled in Html Reader, however, to avoid BC issues, it needs to be explicitly enabled for Html Writer. A new boolean property `dataFormula` and setter are added to Writer; it will default to current behavior `false` for now, but my plan is to change the default in the next breaking release. I cannot think of a decent use case for a getter, so have not added one.
oleibman