Make sure we always use valid urls

[Nyholm]

This is a massive PR but it does nothing but add a slash ("/") infront of all URL.
The reason for this is because Guzzle decided to validate the url as soon as it changes. (guzzle/psr7#94)

We added a url like "user/Nyholm/repos" then we use AddHostPlugin which executes like this:

$uri = $request->getUri()
                ->withHost($this->host->getHost())
                ->withScheme($this->host->getScheme())
                ->withPort($this->host->getPort())
            ;

If we add a host like "api.github.com" it will not be valid at that state. It was before because https://github.com/guzzle/psr7/pull/94/files#diff-db412dbd2c689be753da90d14beffc1bL461

With this PR we make sure to always have a valid URL. Because "/foo" is a valid URL.

[GrahamCampbell]

Sad times...

[Nyholm]

Haha, On my way.
I have to prepend the slash there as well.

[GrahamCampbell]

Yeh. :)

[Nyholm]

Thank you for merging.