asset-manager: Headers
Previously, the Rails app received X-Sendfile-Type & X-Accel-Mapping request headers and set the X-Accel-Redirect response header which caused Rack::Sendfile not to send the asset file in the body of the response. Nginx would interpret the X-Accel-Redirect response header and serve the file directly from disk. When running the app standalone (i.e. without Nginx) the app would not receive the request headers mentioned above. This in turn caused Rack::Sendfile to send the asset file in the body of the response. Thus it was possible to use the app running standalone to serve asset requests.
However, now the app doesn't make use of Rack::Sendfile and instead always responds to asset requests by setting the X-Accel-Redirect response header and not sending the asset file in the response body. Nginx is configured to proxy the request to the URL supplied in X-Accel-Redirect response header on the development VM, but if you want to run the app standalone and you want to actually serve the asset file in the response body, you'll need something (e.g. Nginx) in front of the Rails app.