calculate cell formulas by default

[arteq]

The 'setPreCalculateFormulas(true)' is needed to save the file with calculated forumulas. Without it if you have cell with formula ex. '=A1+A2' or `=SUM(A1:A2)' the result will be always zero until you reenter/recalculate the cell value once opened in Excel. Took me some time to figure this out, I belive it's the desired behavior for 99% developers who use any kind of formulas in generated spreadsheet so I'd make turned on by default.