Simplify HttpHeaders constructor

As of gh-33913, `HttpHeaders` does not implement the `MultiValueMap`
contract anymore, so we can take this opportunity to simplify one of the
constructors to not consider that the argument could be an `HttpHeaders`
instance. This case is already covered by the other constructor.

See gh-33913

Author: bclozel

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

@@ -455,18 +455,7 @@ public HttpHeaders() {
 	 */
 	public HttpHeaders(MultiValueMap<String, String> headers) {
 		Assert.notNull(headers, "MultiValueMap must not be null");
-		if (headers == EMPTY) {
-			this.headers = CollectionUtils.toMultiValueMap(new LinkedCaseInsensitiveMap<>(8, Locale.ENGLISH));
-		}
-		else if (headers instanceof HttpHeaders httpHeaders) {
-			while (httpHeaders.headers instanceof HttpHeaders wrapped) {
-				httpHeaders = wrapped;
-			}
-			this.headers = httpHeaders.headers;
-		}
-		else {
-			this.headers = headers;
-		}
+		this.headers = headers;
 	}
 
 	/**