Incorrect Request-URI when using HttpClient with proxy

[dmromanov]

This is a (multiple allowed):

• bug

• enhancement

• feature-discussion (RFC)

• CakePHP Version: 3.3.12

• Platform and Target:
  PHP CLI 7.1.0-5+deb.sury.org~yakkety+1
  Squid proxy 3.5.12
  Ubuntu 16.10

What you did

I've used Cake\Http\Client to send a request to a web-server via a proxy.

        $request = new Client();

        $request->get('http://127.0.0.1:8765/foo', [], [
            'proxy' => [
                'proxy' => '127.0.0.1:3128',
            ]
        ]);

What happened

Http Client sent a request with an URI specified in relative URI form.
Request failed. Proxy returned an error.

What you expected to happen

According to RFC2616 when sending a request to a proxy request URI MUST be sent in an absolute form.