Configure XML Transformer in a Saxon-compatible manner

Closes gh-615

Author: wilkinsona

spring-restdocs-core/src/main/java/org/springframework/restdocs/payload/XmlContentHandler.java

@@ -173,10 +173,10 @@ private String prettyPrint(Document document) {
 			StringWriter stringWriter = new StringWriter();
 			StreamResult xmlOutput = new StreamResult(stringWriter);
 			TransformerFactory transformerFactory = TransformerFactory.newInstance();
-			transformerFactory.setAttribute("indent-number", 4);
 			Transformer transformer = transformerFactory.newTransformer();
 			transformer.setOutputProperty(OutputKeys.INDENT, "yes");
 			transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
+			transformer.setOutputProperty("{http://xml.apache.org/xslt}indent-amount", "4");
 			transformer.transform(new DOMSource(document), xmlOutput);
 			return xmlOutput.getWriter().toString();
 		}