fix when xml begin with cdata

such as 
$input=[
'#key'=> 'val'
]

Author: dwdcth

ArrayToXML.php

@@ -41,7 +41,7 @@ public function buildXML($data, $startElement = 'data')
         $xml->startDocument($this->version, $this->encoding);
         $xml->startElement($startElement);
 
-        $this->writeAttr($xml, $data);
+        $data = $this->writeAttr($xml, $data);
         $this->writeEl($xml, $data);
 
         $xml->endElement(); //write end element