Make sure the path prefix is actually present even if the request has been altered before

Follow up on php-http#103.

Everything worked fine until I refactored my tests and started to use a singleton API client for offline testing. If I call the same API endpoints in the same test with the client then the first API request gets prefixed properly but the second one does not, because the generated `$identifier` is already exist in the ` $this->alteredRequests`.

I tried to use something else for identifier, I tried different ways to make the identifier more unique but I could not figure out a better solution than this for s start.

Author: mxr576

src/Plugin/AddPathPlugin.php

@@ -48,7 +48,7 @@ public function handleRequest(RequestInterface $request, callable $next, callabl
     {
         $identifier = spl_object_hash((object) $first);
 
-        if (!array_key_exists($identifier, $this->alteredRequests)) {
+        if (!array_key_exists($identifier, $this->alteredRequests) || strpos($request->getUri()->getPath(), $this->uri->getPath()) !== 0) {
             $request = $request->withUri($request->getUri()
                 ->withPath($this->uri->getPath().$request->getUri()->getPath())
             );