[BAEL-11601] Move and update Testing REST with multiple MIME types article code

Moved the code and cleaned spring-rest-full module.

spring-boot-rest/src/test/java/com/baeldung/test/TestMarshallerFactory.java

@@ -27,8 +27,7 @@ public class TestMarshallerFactory implements FactoryBean<IMarshaller> {
             case "json":
                 return new JacksonMarshaller();
             case "xml":
-                // If we need to implement xml marshaller we can include spring-rest-full XStreamMarshaller
-                throw new IllegalStateException();
+                return new XStreamMarshaller();
             default:
                 throw new IllegalStateException();
             }

spring-boot-rest/src/test/java/com/baeldung/test/XStreamMarshaller.java

@@ -0,0 +1,49 @@
+package com.baeldung.test;
+
+import java.util.List;
+
+import com.baeldung.persistence.model.Foo;
+import org.springframework.http.MediaType;
+
+import com.google.common.base.Preconditions;
+import com.thoughtworks.xstream.XStream;
+
+public final class XStreamMarshaller implements IMarshaller {
+
+    private XStream xstream;
+
+    public XStreamMarshaller() {
+        super();
+
+        xstream = new XStream();
+        xstream.autodetectAnnotations(true);
+        xstream.processAnnotations(Foo.class);
+    }
+
+    // API
+
+    @Override
+    public final <T> String encode(final T resource) {
+        Preconditions.checkNotNull(resource);
+        return xstream.toXML(resource);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public final <T> T decode(final String resourceAsString, final Class<T> clazz) {
+        Preconditions.checkNotNull(resourceAsString);
+        return (T) xstream.fromXML(resourceAsString);
+    }
+
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> List<T> decodeList(final String resourcesAsString, final Class<T> clazz) {
+        return this.decode(resourcesAsString, List.class);
+    }
+
+    @Override
+    public final String getMime() {
+        return MediaType.APPLICATION_XML.toString();
+    }
+
+}