undefined method `authenticate' for nil:NilClass While Testing Custom OmniAuth Strategy in Rails With Devise

Question

I wrote a custom OmniAuth strategy and am now trying to write Rspec tests around it, but I keep getting this error:

NoMethodError:
   undefined method `authenticate' for nil:NilClass
 # ./controllers/application_controller.rb:58:in `block in <class:ApplicationController>'
 # ./lib/omniauth/custom_strategy.rb:135:in `block in callback_phase'
 # ./lib/omniauth/custom_strategy.rb:119:in `callback_phase'
 # ./spec/lib/omniauth/custom_strategy_spec.rb:62:in `block (3 levels) in <top (required)>'

After digging through the Devise code, I found that the error was originating in lib/devise/controllers/helpers.rb, where the current_user method gets defined in ApplicationController:

def current_#{mapping}
  @current_#{mapping} ||= warden.authenticate(:scope => :#{mapping})
end

In this case, warden is defined as:

def warden
  request.env['warden']
end

So, in my spec, since it's not a controller spec, I put in custom code to create a Rack app (as seen here: https://github.com/mkdynamic/omniauth/blob/master/spec/omniauth/strategies/developer_spec.rb) like so:

# Outside of the spec block:

RSpec.configure do |config|
  config.include Rack::Test::Methods
end

# Inside the spec block:

let(:app){ Rack::Builder.new do |b|
  b.use Rack::Session::Cookie
  b.use OmniAuth::Strategies::CustomStrategy
  b.run lambda{|env| [200, {}, ['Not Found']]}
end.to_app }

The problem here is that the Rack::Test::Methods don't include a request object, like Rspec provides in its controller tests. So, request.env is not defined and therefore warden is not defined.

I tried including Devise::TestHelpers, as described here (All Ruby tests raising: undefined method `authenticate' for nil:NilClass), but since @request is not defined, it throws an error.

I've also tried many other solutions, but none seem to deal with non-controller tests, since they all rely on a request object. Has anyone had experience with these issues and can shed some light on possible solutions?

Thanks in advance.