Consistently resolve base URL according to HTTP specs

[clue]

This changeset improves the base URL logic to consistently resolve base URL according to HTTP specs (https://tools.ietf.org/html/rfc3986#section-5.2).

// new: will request http://api.example.com/v2/users
$browser = $browser->withBase('http://api.example.com/v3/');
$browser->get('../v2/users')->then(…);

I consider this to be a feature addition and not a BC break because the previous behavior was both unclear and not well-defined. This now follows common URL resolving logic supporting relative references such as ../,./ and others.