Fix sast_artifact method delegation and add some tests

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/serializers/merge_request_entity.rb

@@ -202,7 +202,7 @@ class MergeRequestEntity < IssuableEntity
     end
   end
 
-  expose :sast_path, if: expose_sast_data(mr, current_user) do |merge_request|
+  expose :sast_path, if: -> (mr, _) { expose_sast_data?(mr, current_user) } do |merge_request|
     raw_project_build_artifacts_url(merge_request.source_project,
                                     merge_request.sast_artifact,
                                     path: 'gl-sast-report.json')

ee/app/models/ee/merge_request.rb

@@ -11,7 +11,7 @@ module EE
 
       delegate :codeclimate_artifact, to: :head_pipeline, prefix: :head, allow_nil: true
       delegate :codeclimate_artifact, to: :base_pipeline, prefix: :base, allow_nil: true
-      delegate :sast_artifact, to: :head_pipeline, prefix: :false, allow_nil: true
+      delegate :sast_artifact, to: :head_pipeline, allow_nil: true
     end
 
     def rebase_dir_path

spec/ee/spec/models/ee/merge_request_spec.rb

@@ -166,4 +166,8 @@ describe MergeRequest do
       it { is_expected.to eq(expected) }
     end
   end
+
+  describe '#sast_artifact' do
+    it { is_expected.to delegate_method(:sast_artifact).to(:head_pipeline) }
+  end
 end

spec/ee/spec/serializers/merge_request_entity_spec.rb

+require 'spec_helper'
+
+describe MergeRequestEntity do
+  let(:user) { create(:user) }
+  let(:project) { create :project, :repository }
+  let(:merge_request) { create(:merge_request, source_project: project, target_project: project) }
+  let(:build) { create(:ci_build, name: 'sast') }
+  let(:request) { double('request', current_user: user) }
+
+  subject do
+    described_class.new(merge_request, request: request)
+  end
+
+  it 'has sast data' do
+    allow(subject).to receive(:expose_sast_data?).and_return(true)
+    allow(merge_request).to receive(:sast_artifact).and_return(build)
+
+    expect(subject.as_json).to include(:sast_path)
+  end
+end