Fix MR assignees for push options

With multiple MR assignees, `merge_request.assignee_id` is always nil,
and `merge_request.assignee_ids` isn't in `merge_request.attributes`.
So the existing code doesn't set assignees in the created MR.

This fix gets all the tests passing, but we should also check that no
other associations in the MergeRequest need similar fixups.

app/services/merge_requests/push_options_handler_service.rb

@@ -100,7 +100,7 @@ module MergeRequests
         merge_request = ::MergeRequests::CreateService.new(
           project,
           current_user,
-          merge_request.attributes
+          merge_request.attributes.merge(assignees: merge_request.assignees)
         ).execute
       end
 
@@ -119,7 +119,7 @@ module MergeRequests
 
     def create_params(branch)
       params = {
-        assignee: current_user,
+        assignees: [current_user],
         source_branch: branch,
         source_project: project,
         target_branch: push_options[:target] || target_project.default_branch,

spec/services/merge_requests/push_options_handler_service_spec.rb

@@ -38,7 +38,7 @@ describe MergeRequests::PushOptionsHandlerService do
     it 'assigns the MR to the user' do
       service.execute
 
-      expect(last_mr.assignee).to eq(user)
+      expect(last_mr.assignees).to contain_exactly(user)
     end
 
     context 'when project has been forked' do