Add spec for singular / plural model

spec/testing_record/model_spec.rb

@@ -1,5 +1,27 @@
 # frozen_string_literal: true
 
 RSpec.describe TestingRecord::Model do
-  it { is_expected.to be_a(TestingRecord::Model) }
+  context 'when classified as a singular model' do
+    let(:model) do
+      Class.new(TestingRecord::Model) do
+        type :singular
+      end
+    end
+
+    it 'sets the singular model properties' do
+      expect(model.instance_variable_get(:@type)).to eq(:singular)
+    end
+  end
+
+  context 'when classified as a plural model' do
+    let(:model) do
+      Class.new(TestingRecord::Model) do
+        type :plural
+      end
+    end
+
+    it 'sets the singular model properties' do
+      expect(model.instance_variable_get(:@type)).to eq(:plural)
+    end
+  end
 end