Previously, `ureq` used `HTTP CONNECT` for *all* requests, including
plain-text HTTP requests. However, some proxy servers like Squid
only allow tunneling via the `HTTP CONNECT` method on port 443 - since
it is usually only used to proxy HTTPS requests. As a result,
it was not possible to use `ureq` with a Squid server in its default configuration.
With the changes from this commit, ureq will interact with HTTP proxies
in a more standard-conforming way, where `CONNECT` is only used for
HTTPS requests. HTTP request paths are transformed in such a way that they
comply with RFC 7230 [1].
Tested against Squid 4.13 in standard configuration, with and without basic authentication,
for HTTP and HTTPS requests.
[1] https://www.rfc-editor.org/rfc/rfc7230#section-5.3.2
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