Html Reader recognizes a `data-format` attribute for storing the number format associated with a cell. However, Html Writer does not set this attribute. This PR offers a new Writer property `preserveFormatAndValue` and setter; when `true`, for any cell which uses a number format other than the default `General` the writer will generate a `data-format` attribute and, for strings or numbers, it will also generate a `data-value` attribute. This will increase the accuracy of the spreadsheet when reading/writing Html.
To make this change less abstract, assume a cell which has a value of `1234` and a number format of `$#,###.00`. The cell displayed in the generated Html will appear, regardless of the setting of the new property, as `$1,234.00`. However, the Reader currently treats it as a string with that value. If the html has been generated with the new property set to `true`, the Reader will treat it as a numeric cell with a value of `1234` and a style of `#,###.00`. This permits a more accurate reproduction of the original spreadsheet.
I suspect that setting the new property, aside from causing a slight increase in file size, will not cause any significant breaks. However, I'm not totally convinced, so the property will default to `false` (no change in behavior). The default may change in a future breaking release.
oleibman