> You do not say further why, you just state that. I find this pretty poisonous for a critical review of the proposal because this sounds like it's forbidding to think about certain changes.

I apologize. It's hard to have a discussion with people when almost every point being brought up is an opinion. I could have stated that better:

Providing a HeaderValuesInterface for header field values that is a fake array that provides no additional benefits like being able to be cast to a string provides little to no value and is no more convenient to use than just providing an array. If people suggest removing __toString() from everything, including this interface, then it makes this interface pointless.

> Perhaps write some tests that verfiy that all features are actually needed for interacting with HTTP messages?

Just FYI, https://github.com/guzzle/guzzle/tree/guzzle-psr-http/src/Guzzle/Http/Message