Fix bold/italic format when a cell is string (not object).

The code edit makes 'cell' an object before the ibold/italic property
checks are made in the style object assignment below. If 'cell' is
left as a string (pre-edit), then cell.bold/italic are functions,
not undefined as expected. When 'style' is checked later, bold+italic
are mistakenly turned on for such cells.
--
I looked at adding a test to show this behavior, but would have had to add a great deal of infrastructure. As an alternative, I investigated whether the same change was needed and testable in https://github.com/stephen-hardy/xlsx.js.  That code looks the same, but the case is tested and seems to work. I don't understand why the same issue does not occur.