if you are json
application/atom+json;charset=utf-8
if your xml means try this
$headers = array('Content-Type: application/atom+xml; charset=utf-8',
'Content-Length: '. strlen($xml),
'Authorization: GoogleLogin '. trim($authToken)),
'Connection: close');
or
check your xml document tag
<!-- Correct -->
<?xml version="1.0" encoding="utf-8"?>