fix for input URL with no path

src/p3k/HTTP/Test.php

@@ -58,7 +58,7 @@ class Test implements Transport {
     $parts = parse_url($url);
     if($parts['path']) {
       $parts['path'] = '/'.str_replace('/','_',substr($parts['path'],1));
-      if($parts['path'] == '/')
+      if(!isset($parts['path']) || $parts['path'] == '/')
         $parts['path'] = '/_';
       $filepathurl = self::_build_url($parts);
     }