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

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

@@ -136,4 +136,14 @@ class HttpEntityTests {
 				.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);
+
+	}
+
 }