Fix formula support in Xlsx style

[Yugam18]

[pietersv]

@yugam This is a useful fix. We'll aim to incorporate this if we open the branch for new development. Generally for changes we need to make sure that:

• changes are to the source code in the /bits/ directory rather than to xlsx.js (which is generated and overwritten by the make process)
• add a unit test to verify this works
• verify the test suite passes