[Polish here](https://github.com/garyrussell/spring-integration/commit/baa03a78c0200a5bd5941b5e40a70ddc1d57b419) and [without whitespace changes here](https://github.com/garyrussell/spring-integration/commit/baa03a78c0200a5bd5941b5e40a70ddc1d57b419?w=1).

I want you to check it because I was not clear why you said "...headers have more freedom..." when it could be a header or payload expression.