Pass 418 status through munge

The munge process is about to be refactored, so the aim of this commit is to pass the 418 status from the document_mappings.csv through to the data/mappings/*.csv in the easiest way.

This works, and the existing tests all pass, but I'd like you to take a look at it, particularly the Ruby changes I've made, in case you see something obvious I've missed.