Fix decode_rack_response

The Rack response body is _not_ an Enumerable, it just responds to #each. The method was assuming otherwise, which led to breakage when trying to use appsignal/appsignal-ruby#1037

Author: julik

lib/rails_twirp/testing/integration_test.rb

@@ -91,7 +91,7 @@ def request_content_type
     end
 
     def decode_rack_response(service, rpc, status, headers, body)
-      body = body.join # body is an Enumerable
+      body = Array.wrap(body).join # body is each-able
 
       if status === 200
         output_class = service.rpcs[rpc][:output_class]