Breaking change not marked as breaking in changelog #700

Hi!

I just upgraded the HTTPrb gem and had some issues. This PR #576 introduces a breaking change but it's not noted in the changelog as such (unless I'm missing something).
Previously, the headers were normalized when they were added so x-total-count would be stored as X-Total-Count. We then used response.to_a which returns [status, headers_as_hash, body]. Before this change, the hash keys were already normalized so when we did headers["X-Total-Count"] it worked. After this change though, it started returning nil since the hash keys didn't match anymore.

Perhaps we can update the changelog to avoid other people hitting the same issues as me? :)