Add spotless plugin and fix grammar issues

Author: thboileau

org.restlet.ext.xml/src/main/java/org/restlet/ext/xml/SaxRepresentation.java

@@ -135,8 +135,8 @@ public InputSource getInputSource() throws IOException {
     @Override
     public SAXSource getSaxSource() throws IOException {
         if (this.source == null && this.xmlRepresentation != null) {
-            if (xmlRepresentation instanceof XmlRepresentation) {
-                this.source = ((XmlRepresentation) xmlRepresentation).getSaxSource();
+            if (xmlRepresentation instanceof XmlRepresentation xmlRepresentationCast) {
+                this.source = xmlRepresentationCast.getSaxSource();
             } else {
                 try {
                     SAXParserFactory spf = SAXParserFactory.newInstance();