* Handle Error in Formula Processing Better for Xls
When there is an error writing a formula to an Xls file,
which seems to happen when a defined name is part of a formula,
the cell is currently left blank. A better result would be to
write the calculated value of the formula.
* Making Changes Suggested in Review
Per comment from Mark Baker:
1. Made return codes from writeFormula into constants.
2. Skipped redundant call to getCellValue when possible.
3. Added support for bool type, adding additional tests
for bool and string.
Per comment from PowerKiki:
1. Used standardized convention for assigning file name in test.
Since before this change, save would throw Exception, I kept
the unlink for the file in tearDown.
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