Increase Budget, Heading and Group model specs to cover slug uniqueness

Why:

* Slug must be unique among:
 1. Budget slug: among other budgets
 2. Group slug: among other groups from its budget
 3. Heading slug: among other headings from all the groups from its budget

How:

* Adding checks for all possible scenarios on each model specs

spec/models/budget/group_spec.rb

@@ -4,4 +4,20 @@ describe Budget::Group do
 
   it_behaves_like "sluggable"
 
+  let(:budget) { create(:budget) }
+
+  describe "name" do
+    before do
+      create(:budget_group, budget: budget, name: 'object name')
+    end
+
+    it "can be repeatead in other budget's groups" do
+      expect(build(:budget_group, budget: create(:budget), name: 'object name')).to be_valid
+    end
+
+    it "must be unique among all budget's groups" do
+      expect(build(:budget_group, budget: budget, name: 'object name')).not_to be_valid
+    end
+  end
+
 end

spec/models/budget/heading_spec.rb

@@ -4,4 +4,25 @@ describe Budget::Heading do
 
   it_behaves_like "sluggable"
 
+  let(:budget) { create(:budget) }
+  let(:group) { create(:budget_group, budget: budget) }
+
+  describe "name" do
+    before do
+      create(:budget_heading, group: group, name: 'object name')
+    end
+
+    it "can be repeatead in other budget's groups" do
+      expect(build(:budget_heading, group: create(:budget_group), name: 'object name')).to be_valid
+    end
+
+    it "must be unique among all budget's groups" do
+      expect(build(:budget_heading, group: create(:budget_group, budget: budget), name: 'object name')).not_to be_valid
+    end
+
+    it "must be unique among all it's group" do
+      expect(build(:budget_heading, group: group, name: 'object name')).not_to be_valid
+    end
+  end
+
 end

spec/models/budget_spec.rb

@@ -4,6 +4,16 @@ describe Budget do
 
   it_behaves_like "sluggable"
 
+  describe "name" do
+    before do
+      create(:budget, name: 'object name')
+    end
+
+    it "is validated for uniqueness" do
+      expect(build(:budget, name: 'object name')).not_to be_valid
+    end
+  end
+
   describe "description" do
     it "changes depending on the phase" do
       budget = create(:budget)