calling change-Method multiple times fails

I hope you get my point:
There is something wrong within the CalDAVClient.php.

When I call the change-Method multiple times, is uses an old If-Match-Header ID.
The If-Match-Header needs to be resetted each time.

I solved this by adding a 
`$this->headers['match'] = "";` at line 396 in CalDAVClient.php
 