🐛 Bug Report

The implementation of isEqual is bad. It only looks at keys of a and ignores the keys of b.

Lib version: 4.3.0

Steps To Reproduce

I was exporting an xlsx and I noticed that a column was interpreting numbers as dates. I discovered that the toModel method in column.js merges the column with the previous column because the column has an empty style. This causes column.equivalentTo(col) to return true at this line: https://github.com/exceljs/exceljs/blob/master/lib/doc/column.js#L275.

The check boils down to: _.isEqual(this.style, other.style). In my case, this.style is empty, so it can be written as: _.isEqual({}, {...}). isEqual will enumerate the keys of the first argument, which is empty, so it doesn't actually check any key.

Possible solution (optional, but very helpful):

Check that the keys match before iterating on a.