Add controller test for merge requests pipelines API

app/serializers/pipeline_serializer.rb

 class PipelineSerializer < BaseSerializer
-  entity PipelineEntity
   class InvalidResourceError < StandardError; end
   include API::Helpers::Pagination
   Struct.new('Pagination', :request, :response)
 
+  entity PipelineEntity
+
   def represent(resource, opts = {})
     if paginated?
       raise InvalidResourceError unless resource.respond_to?(:page)

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -668,9 +668,40 @@ describe Projects::MergeRequestsController do
   end
 
   describe 'GET pipelines' do
+    before do
+      create(:ci_pipeline, project: merge_request.source_project,
+                           ref: merge_request.source_branch,
+                           sha: merge_request.diff_head_sha)
+    end
+
+    context 'when using HTML format' do
       it_behaves_like "loads labels", :pipelines
     end
 
+    context 'when using JSON format' do
+      before do
+        get :pipelines,
+            namespace_id: project.namespace.to_param,
+            project_id: project.to_param,
+            id: merge_request.iid,
+            format: :json
+      end
+
+      let(:json_response) { JSON.parse(response.body) }
+
+      it 'responds with a rendered HTML partial' do
+        expect(response)
+          .to render_template('projects/merge_requests/show/_pipelines')
+        expect(json_response).to have_key 'html'
+      end
+
+      it 'responds with serialized pipelines' do
+        expect(json_response).to have_key 'pipelines'
+        expect(json_response['pipelines']).not_to be_empty
+      end
+    end
+  end
+
   describe 'GET conflicts' do
     let(:json_response) { JSON.parse(response.body) }