Allow start_date as only parameter when updating Milestone via API

lib/api/milestone_responses.rb

@@ -24,7 +24,7 @@ module API
           optional :state_event, type: String, values: %w[close activate],
                                  desc: 'The state event of the milestone '
           use :optional_params
-          at_least_one_of :title, :description, :due_date, :state_event
+          at_least_one_of :title, :description, :start_date, :due_date, :state_event
         end
 
         def list_milestones_for(parent)

spec/support/api/milestones_shared_examples.rb

@@ -196,6 +196,12 @@ shared_examples_for 'group and project milestones' do |route_definition|
 
       expect(json_response['state']).to eq('closed')
     end
+
+    it 'updates milestone with only start date' do
+      put api(resource_route, user), start_date: Date.tomorrow
+
+      expect(response).to have_gitlab_http_status(200)
+    end
   end
 
   describe "GET #{route_definition}/:milestone_id/issues" do