Migrating from Gun 1.1 to 1.2

Gun 1.2 adds support for the CONNECT request over HTTP/1.1 connections.

Features added

CONNECT requests can now be issued on HTTP/1.1 connections. The tunneled connection can use any of the protocols Gun supports: HTTP/1.1, HTTP/2 and Websocket over both TCP and TLS transports. Note that Gun currently does not support tunneling a TLS connection over a TLS connection due to limitations in Erlang/OTP.
Gun supports sending multiple CONNECT requests, allowing the tunnel to the origin server to go through multiple proxies.
Gun supports sending CONNECT requests with authorization credentials using the Basic authentication mechanism.
Update Cowlib to 2.6.0

The functions gun:connect/2,3,4 have been added. They can be used to initiate CONNECT requests on HTTP/1.1 connections.