Have pick_avatar return json.

I'm working on writing a test in the discourse_api gem for uploading
avatars and the pick method needs to return a json response.

I also added a test to make sure json is returned.

app/controllers/users_controller.rb

@@ -500,7 +500,7 @@ class UsersController < ApplicationController
     end
     user.save!
 
-    render nothing: true
+    render json: success_json
   end
 
   def destroy_user_image