+ var len
+ var body = self.body
+ if body isa Text then
+ len = body.byte_length
+ else if body isa Bytes then
+ len = body.length
+ else
+ # We need the length, but there is no length in a writable.
+ # So just render it as a bytes then measure :/
+ body = body.write_to_bytes
+ len = body.length
+ # Keep the body as bytes since we have it
+ self.body = body
+ end