XMLWriter: Change setIndent as it expects a boolean (#5011)

Author: staabm

reference/xmlwriter/xmlwriter/setindent.xml

@@ -78,7 +78,7 @@
 <?php
 $writer = new XMLWriter();
 $writer->openMemory();
-$writer->setIndent(2);
+$writer->setIndent(true);
 $writer->startDocument();
 $writer->startElement('p');
 $writer->text('before');