Disable "strings to Formula" by default

[randomstuff]

By default, XlsxWriter expands cells starting with = into formulas. Having this behavior happen by default is dangerous as it makes it easy to write code which is vulnerable to formula injections. Wouldn't it be safer to make this an opt-in instead?

See pandas-dev/pandas#29095

[jmcnamara]

Converting strings to formulas in write() is the documented behaviour and has been for 6 years. Changing it now would break that functionality for a large number of users, so I'm not going to do that.

This behaviour is easily turned off in the constructor:

workbook = xlsxwriter.Workbook(filename, {'strings_to_formulas': False})