Remove the yielding of anchors in the header callback

Hello,

This pull request basically reverts facd5c5a. We should not change any method's arity between point releases. I guess there is no easy solution here. The only option I'm seeing is to define an `anchor` method that the `header` callback would rely on but for now, we should include it in 3.2 I think.

I also took the liberty to create a `3-1-branch` branch as `master` seems more to target 3.2.0 than 3.1.2.

Fixes #352.

Have a nice day.