Support canEncode() for JAXBElement in Jaxb2XmlEncoder

Commit d7970e4 introduced support for JAXBElement in
Jaxb2XmlEncoder's encodeValue() method; however, canEncode() still
returned false for a JAXBElement element type.

This commit revises canEncode() so that it returns true for an element
type that is assignable from JAXBElement.

See gh-30552
See gh-32972
Closes gh-32977

Author: deblockt

spring-web/src/main/java/org/springframework/http/codec/xml/Jaxb2XmlEncoder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2023 the original author or authors.
+ * Copyright 2002-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -92,7 +92,8 @@ public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType
 		if (super.canEncode(elementType, mimeType)) {
 			Class<?> outputClass = elementType.toClass();
 			return (outputClass.isAnnotationPresent(XmlRootElement.class) ||
-					outputClass.isAnnotationPresent(XmlType.class));
+					outputClass.isAnnotationPresent(XmlType.class) ||
+					elementType.isAssignableFrom(JAXBElement.class));
 		}
 		else {
 			return false;

spring-web/src/test/java/org/springframework/http/codec/xml/Jaxb2XmlEncoderTests.java

@@ -63,6 +63,8 @@ protected void canEncode() {
 		assertThat(this.encoder.canEncode(forClass(TypePojo.class), MediaType.APPLICATION_XML)).isTrue();
 		assertThat(this.encoder.canEncode(forClass(getClass()), MediaType.APPLICATION_XML)).isFalse();
 
+		assertThat(this.encoder.canEncode(forClass(JAXBElement.class), MediaType.APPLICATION_XML)).isTrue();
+
 		// SPR-15464
 		assertThat(this.encoder.canEncode(ResolvableType.NONE, null)).isFalse();
 	}