Make HttpEntity headers mutables

Since its inception, instantiating an `HttpEntity` makes its
`HttpHeaders` read-only. While immutability is an interesting design
principle, here we shouldn't enforce this.

For example, developers can expect to instantiate a `ResponseEntity`
and still mutate its headers.

Closes gh-35888

Author: bclozel

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

@@ -102,7 +102,7 @@ public HttpEntity(HttpHeaders headers) {
 	 */
 	public HttpEntity(@Nullable T body, @Nullable HttpHeaders headers) {
 		this.body = body;
-		this.headers = HttpHeaders.readOnlyHttpHeaders(headers != null ? headers : new HttpHeaders());
+		this.headers = (headers != null) ? headers : new HttpHeaders();
 	}
 
 	/**
@@ -123,8 +123,7 @@ public HttpEntity(MultiValueMap<String, String> headers) {
 	 */
 	@Deprecated(since = "7.0", forRemoval = true)
 	public HttpEntity(@Nullable T body, @Nullable MultiValueMap<String, String> headers) {
-		this.body = body;
-		this.headers = HttpHeaders.readOnlyHttpHeaders(headers != null ? new HttpHeaders(headers) : new HttpHeaders());
+		this(body, (headers != null) ? new HttpHeaders(headers) : new HttpHeaders());
 	}
 
 

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

@@ -136,4 +136,14 @@ void mutateEmptyInstanceHeaders() {
 				.isEmpty();
 	}
 
+	@Test
+	void headerAreMutable() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.TEXT_PLAIN);
+		String body = "foo";
+		HttpEntity<String> httpEntity = new HttpEntity<>(body, headers);
+		httpEntity.getHeaders().setContentType(MediaType.APPLICATION_JSON);
+
+	}
+
 }