Refactor zonefile_setting's origin list

#186

spec/models/zonefile_setting_spec.rb

@@ -0,0 +1,17 @@
+require 'rails_helper'
+
+RSpec.describe ZonefileSetting, db: false do
+  it 'has versions' do
+    expect(described_class.new.versions).to eq([])
+  end
+
+  describe '::origins' do
+    before :example do
+      expect(described_class).to receive(:pluck).with(:origin).and_return('origins')
+    end
+
+    it 'returns origins' do
+      expect(described_class.origins).to eq('origins')
+    end
+  end
+end