Rails: switch to puma for request specs

We are working on a rails 3.2 app with RSpec 3.7 and want to use puma as web server for our rails request specs.

We know we can switch the server for Capybara feature specs using

Capybara.register_server :puma

But how can we switch the server for request specs?

Background:

We want to spec concurrency issues and need our test server to actually process requests parallelly.