add expectation that overriding the config stores the expected instance in env

Author: oreoshake

spec/lib/secure_headers/middleware_spec.rb

@@ -33,6 +33,7 @@ module SecureHeaders
       end
       request = Rack::Request.new({})
       SecureHeaders.use_secure_headers_override(request, "my_custom_config")
+      expect(request.env[SECURE_HEADERS_CONFIG]).to be(Configuration.get("my_custom_config"))
       _, env = middleware.call request.env
       expect(env[CSP::HEADER_NAME]).to match("example.org")
     end