header[Content-Length] doesn't match actual content length

The middleware replace the response body after it has been generated, and in some case, `header[Content-Length]` has already been set.

The placeholder are longer then the actual token (`token` length are `44`, while the placeholder length are `47`), hence, the number given by `header[Content-Length]` becomes invalid.

This PR tried to fix this by using a placeholder with 44 character.