use json as default response format

spec/routing/posts_routing_spec.rb

@@ -3,16 +3,16 @@ require "rails_helper"
 RSpec.describe PostsController, type: :routing do
   describe "routing" do
     it "routes to #index" do
-      expect(get: "/posts").to route_to("posts#index")
+      expect(get: "/posts").to route_to("posts#index", format: :json)
     end
 
     it "routes to #show" do
-      expect(get: "/posts/1").to route_to("posts#show", id: "1")
+      expect(get: "/posts/1").to route_to("posts#show", id: "1", format: :json)
     end
 
 
     it "routes to #create" do
-      expect(post: "/posts").to route_to("posts#create")
+      expect(post: "/posts").to route_to("posts#create", format: :json)
     end
   end
 end