HttpEntity.EMPTY should be immutable

See gh-34812

Signed-off-by: Yanming Zhou <zhouyanming@gmail.com>

Author: quaff

spring-web/src/main/java/org/springframework/http/HttpEntity.java

@@ -49,6 +49,7 @@
  *
  * @author Arjen Poutsma
  * @author Juergen Hoeller
+ * @author Yanming Zhou
  * @since 3.0.2
  * @param <T> the body type
  * @see org.springframework.web.client.RestTemplate
@@ -60,7 +61,7 @@ public class HttpEntity<T> {
 	/**
 	 * The empty {@code HttpEntity}, with no body or headers.
 	 */
-	public static final HttpEntity<?> EMPTY = new HttpEntity<>();
+	public static final HttpEntity<?> EMPTY = new HttpEntity<>(HttpHeaders.EMPTY);
 
 
 	private final HttpHeaders headers;

spring-web/src/test/java/org/springframework/http/HttpEntityTests.java

@@ -27,6 +27,7 @@
 
 /**
  * @author Arjen Poutsma
+ * @author Yanming Zhou
  */
 class HttpEntityTests {
 
@@ -123,4 +124,11 @@ void requestEntity() {
 		assertThat(requestEntity2).isEqualTo(requestEntity);
 	}
 
+	@Test
+	void emptyHttpEntityShouldBeImmutable() {
+		HttpHeaders newHeaders = new HttpHeaders(HttpEntity.EMPTY.getHeaders());
+		newHeaders.add("Authorization", "Bearer some-token");
+		assertThat(HttpEntity.EMPTY.getHeaders().headerNames()).isEmpty();
+	}
+
 }