Fix stubbing on method defined in a prepended module

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/presenters/merge_request_presenter_spec.rb

@@ -392,7 +392,7 @@ describe MergeRequestPresenter do
     before do
       allow(resource).to receive(:source_branch_exists?) { source_branch_exists }
 
-      allow_any_instance_of(::Gitlab::UserAccess)
+      allow_any_instance_of(Gitlab::UserAccess::RequestCacheExtension)
         .to receive(:can_push_to_branch?)
         .with(resource.source_branch)
         .and_return(can_push_to_branch)
@@ -462,7 +462,7 @@ describe MergeRequestPresenter do
       allow(resource).to receive(:rebase_in_progress?) { rebase_in_progress }
       allow(resource).to receive(:should_be_rebased?) { should_be_rebased }
 
-      allow_any_instance_of(::Gitlab::UserAccess)
+      allow_any_instance_of(Gitlab::UserAccess::RequestCacheExtension)
         .to receive(:can_push_to_branch?)
         .with(resource.source_branch)
         .and_return(can_push_to_branch)