Description
Same as #164. On Symfony, we've got symfony/symfony#32819 a few months ago.
TL;DR there are API servers that are case sensitive. Yes, this is not HTTP-compliant.
Yet, lowercasing the headers before sending them is not required.
We chose to not lowercase headers when sending them on HTTP/1.x so that the client can still consume those APIs.
I think the Amp client should reconsider and do the same. At least on Symfony's side, the current strict behavior means we're not sure if we can ship Amp's implementation as the default one (when it is installed).