fixed api cors 使用rack-cors，解决跨域问题  #233

使用rack-cors，解决跨域问题，实际中也是这样用，不过application.rb里配置的路由还需要更严谨

```
config.middleware.use Rack::Cors do
      allow do
        origins '*'
        resource '/*', headers: :any, methods: [:get, :post, :put, :delete, :destroy]
      end
    end
```