Add spec for poll model

spec/models/poll_spec.rb

@@ -1,5 +1,23 @@
+# frozen_string_literal: true
 require 'rails_helper'
 
-RSpec.describe Poll, type: :model do
-  pending "add some examples to (or delete) #{__FILE__}"
+RSpec.describe Poll do
+  describe 'validations' do
+    context 'when valid' do
+      let(:poll) { Fabricate.build(:poll) }
+
+      it 'is valid with valid attributes' do
+        expect(poll).to be_valid
+      end
+    end
+
+    context 'when not valid' do
+      let(:poll) { Fabricate.build(:poll, expires_at: nil) }
+
+      it 'is invalid without an expire date' do
+        poll.valid?
+        expect(poll).to model_have_error_on_field(:expires_at)
+      end
+    end
+  end
 end