This one needs to be `validates`: when I add this test it fails with `validate` (albeit with an unhelpful message):

    it { should ensure_inclusion_of(:state).in_array(MappingsBatch::PROCESSING_STATES) }

...but passes with `validates`.

We really should fix this for `Host` as well - I think I got distracted at the point of writing the validation tests for that a week or two ago... Rails does make this a stupidly easy mistake to make.