Add spec for Project#merge_method

It should be deleted from EE side when doing CE->EE merge!

spec/models/project_spec.rb

@@ -408,6 +408,18 @@ describe Project do
     end
   end
 
+  describe '#merge_method' do
+    it 'returns "ff" merge_method when ff is enabled' do
+      project = build(:project, merge_requests_ff_only_enabled: true)
+      expect(project.merge_method).to be :ff
+    end
+
+    it 'returns "merge" merge_method when ff is disabled' do
+      project = build(:project, merge_requests_ff_only_enabled: false)
+      expect(project.merge_method).to be :merge
+    end
+  end
+
   describe '#repository_storage_path' do
     let(:project) { create(:project, repository_storage: 'custom') }