Add specs for updating issuable labels in API

This is in preparation for upgrading to Grape v1.3.x. Parameters of
Array types will no longer automatically coerced to empty arrays if a
key is provided with a nil value.

spec/requests/api/issues/issues_spec.rb

@@ -834,6 +834,12 @@ describe API::Issues do
     end
   end
 
+  describe 'PUT /projects/:id/issues/:issue_id' do
+    it_behaves_like 'issuable update endpoint' do
+      let(:entity) { issue }
+    end
+  end
+
   describe 'DELETE /projects/:id/issues/:issue_iid' do
     it 'rejects a non member from deleting an issue' do
       delete api("/projects/#{project.id}/issues/#{issue.iid}", non_member)

spec/requests/api/merge_requests_spec.rb

@@ -1654,6 +1654,12 @@ describe API::MergeRequests do
     end
   end
 
+  describe 'PUT /projects/:id/merge_reuests/:merge_request_iid' do
+    it_behaves_like 'issuable update endpoint' do
+      let(:entity) { merge_request }
+    end
+  end
+
   describe "POST /projects/:id/merge_requests/:merge_request_iid/context_commits" do
     let(:merge_request_iid)  { merge_request.iid }
     let(:authenticated_user) { user }

spec/support/shared_examples/requests/api/issuable_update_shared_examples.rb

+# frozen_string_literal: true
+
+RSpec.shared_examples 'issuable update endpoint' do
+  let(:area) { entity.class.name.underscore.pluralize }
+
+  describe 'PUT /projects/:id/issues/:issue_id' do
+    let(:url) { "/projects/#{project.id}/#{area}/#{entity.iid}" }
+
+    it 'clears labels when labels param is nil' do
+      put api(url, user), params: { labels: 'label1' }
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response['labels']).to contain_exactly('label1')
+
+      put api(url, user), params: { labels: nil }
+
+      expect(response).to have_gitlab_http_status(:ok)
+      json_response = Gitlab::Json.parse(response.body)
+      expect(json_response['labels']).to be_empty
+    end
+
+    it 'updates the issuable with labels param as array' do
+      stub_const("Gitlab::QueryLimiting::Transaction::THRESHOLD", 110)
+
+      params = { labels: ['label1', 'label2', 'foo, bar', '&,?'] }
+
+      put api(url, user), params: params
+
+      expect(response).to have_gitlab_http_status(:ok)
+      expect(json_response['labels']).to include 'label1'
+      expect(json_response['labels']).to include 'label2'
+      expect(json_response['labels']).to include 'foo'
+      expect(json_response['labels']).to include 'bar'
+      expect(json_response['labels']).to include '&'
+      expect(json_response['labels']).to include '?'
+    end
+  end
+end