Add sluggable shared example for Sluggable concern

Why:

* We need certain models that use a Sluggable to generate a slug based on name attribute on save

How:

* Creating a shared example that knows how to create a factory of the described class
* Checking in it that new objects of the described class get the correct slug

spec/models/concerns/sluggable.rb

@@ -0,0 +1,16 @@
+require 'spec_helper'
+
+shared_examples_for 'sluggable' do
+
+  describe 'generate_slug' do
+    before do
+      create(described_class.name.parameterize.tr('-', '_').to_sym, name: "Marlo Brañido Carlo")
+    end
+
+    context "when a new sluggable is created" do
+      it "gets a slug string" do
+        expect(described_class.last.slug).to eq("marlo-branido-carlo")
+      end
+    end
+  end
+end