Proxy: Only use HTTP CONNECT for HTTPS requests

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

src/lib.rs

@@ -225,14 +225,14 @@
 //!
 //! # Proxying
 //!
-//! ureq supports two kinds of proxies,  HTTP [`CONNECT`], [`SOCKS4`] and [`SOCKS5`], the former is
+//! ureq supports two kinds of proxies,  [`HTTP`], [`SOCKS4`] and [`SOCKS5`], the former is
 //! always available while the latter must be enabled using the feature
 //! `ureq = { version = "*", features = ["socks-proxy"] }`.
 //!
 //! Proxies settings are configured on an [Agent] (using [AgentBuilder]). All request sent
 //! through the agent will be proxied.
 //!
-//! ## Example using HTTP CONNECT
+//! ## Example using HTTP
 //!
 //! ```rust
 //! fn proxy_example_1() -> std::result::Result<(), ureq::Error> {