Add to Budget Budget::Heading Budget::Group specs the sluggable shared example

Why:

* Those classes have a slug attribute that needs to be filled

How:

* Just adding the shared example to Budget model spec, and creating the model spec file for Heading and Group to include it as well

spec/models/budget_spec.rb

@@ -2,6 +2,8 @@ require 'rails_helper'
 
 describe Budget do
 
+  it_behaves_like "sluggable"
+
   describe "description" do
     it "changes depending on the phase" do
       budget = create(:budget)