Add failing spec for updating translated slugs

spec/friendly_id/mobility_spec.rb

@@ -36,6 +36,19 @@
           expect(journalist.slug_en).to be_nil
         end
       end
+
+      # Ref: https://github.com/shioyama/friendly_id-mobility/issues/10
+      it "regenerates slug records for all locales with present values" do
+        article = Article.new(title_en: "English title", title_es: "Título español")
+        article.save
+
+        article.slug_en = nil
+        article.slug_es = nil
+        article.save
+
+        expect(article.slug_en).to eq("english-title")
+        expect(article.slug_es).to eq("titulo-espanol")
+      end
     end
 
     describe "#to_param" do