Keep HTTP/1.0 CONNECT tunnel sockets open

[nallwhy]

Summary

Fix HTTPS tunnel establishment through proxies that return a successful HTTP/1.0 response to CONNECT, such as Tinyproxy.

A successful CONNECT response switches the underlying connection into tunnel mode. However, after the CONNECT response framing changes in #493, Mint finalized the response using the normal HTTP/1.0 persistence rules.

Because HTTP/1.0 connections are non-persistent by default, this closed the underlying TCP socket before Mint attempted the TLS upgrade. On OTP 29, upgrading the closed socket resulted in:

%Mint.TransportError{reason: {:dtls_upgrade, :notsup}}

Changes

• Keep the underlying socket open after any successful CONNECT response.

• Do not apply the response HTTP version or Connection header persistence rules once a tunnel has been established.

• Add a unit test verifying that an HTTP/1.0 200 response to CONNECT leaves the socket open.

• Add an end-to-end regression test using a Tinyproxy-style response:

  HTTP/1.0 200 Connection established
  Proxy-agent: tinyproxy/1.11.1