.flatten to fix more than 1 error
.flatten to fix more than 1 error

`@order` had an internal array.
Instead of a clean array like `["c", "a", "b", "e", "d"]` -
it was returning `["c", "a", ["b", "e", "d"]]`.

This was caused by `any_missing` returning an array then being pushed
into `@order`. When `.map` is called on this and then mutated on by
finding the corresponding worksheet, it only returned the first value
in the internal array. `#flatten` fixes this.

ndoit/fenrir,ndoit/fenrir,ndoit/fenrir,ndoit/fenrir