[BUGFIX] Append slash to folder identifiers

This patch fixes the problem that fetching a folder via API
and using that to upload a file put the file into the wrong folder:

    $folder = $storage->getFolder('myfolder');
    $storage->addFile('/tmp/file.txt', $folder, 'file.txt');
    //created "/myfolderfile.txt" instead of "/myfolder/file.txt"

TYPO3's own
`AbstractHierarchicalFilesystemDriver::canonicalizeAndCheckFolderIdentifier`
appends slashes,
and our `createFolder()` already returned identifiers with slashes at the end.

Resolves: #146

Classes/Driver/AmazonS3Driver.php

@@ -786,7 +786,7 @@ public function getFolderInfoByIdentifier($folderIdentifier)
         $this->normalizeIdentifier($folderIdentifier);
 
         return [
-            'identifier' => $folderIdentifier,
+            'identifier' => rtrim($folderIdentifier, '/') . '/',
             'name' => basename(rtrim($folderIdentifier, '/')),
             'storage' => $this->storageUid,
             'mtime' => null,
@@ -1381,7 +1381,7 @@ protected function getFolder($identifier)
             return $this->getRootLevelFolder();
         }
         $this->normalizeIdentifier($identifier);
-        return new Folder($this->getStorage(), $identifier, basename(rtrim($identifier, '/')));
+        return new Folder($this->getStorage(), rtrim($identifier, '/') . '/', basename(rtrim($identifier, '/')));
     }
 
     /**