The AddHost plugin should take care of the base path

With the following PHP code example:

```php
$httpClient = new HttpMethodsClient(
    new PluginClient(HttpClientDiscovery::find(), [
        new AddHostPlugin(UriFactoryDiscovery::find()->createUri('https://logs.domain.com/api')),
        new ErrorPlugin(),
    ]),
    MessageFactoryDiscovery::find()
);
$httpClient->get('/users');
```

The https://logs.domain.com/users URL will be called instead of /api/users.

The base path should be kept on this plugin.

Author: soullivaneuh

CHANGELOG.md

@@ -11,7 +11,7 @@
 
 - Fix Emulated Trait to use Http based promise which respect the HttpAsyncClient interface
 - RedirectPlugin: use the full URL instead of the URI to properly keep track of redirects
-
+- AddHostPlugin now keeps the base path of the URI
 
 ## 1.2.1 - 2016-07-26
 

src/Plugin/AddHostPlugin.php

@@ -56,6 +56,7 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
                 ->withHost($this->host->getHost())
                 ->withScheme($this->host->getScheme())
                 ->withPort($this->host->getPort())
+                ->withPath($this->host->getPath().$request->getUri()->getPath())
             ;
 
             $request = $request->withUri($uri);