WebClientException should allow access to status code of the response

This commit changes the WebClient so that it now throws a
`WebClientResponseException` for `ResponseSpec.bodyTo`. This newly
introduces exception contains the status code, headers, and body of the
response message.

As a consequence of the above, we had to change `onStatus` so that the
`exceptionFunction` now returns a `Mono<Throwable>` rather than a
`Throwable`, which it was before. The Mono allows for asynchronous
operations, such as reading the contents of the body.

Issue: SPR-15824

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

@@ -73,6 +73,10 @@ public class HttpHeaders implements MultiValueMap<String, String>, Serializable
 
 	private static final long serialVersionUID = -8578554704772377436L;
 
+	/**
+	 * The empty {@code HttpHeaders} instance (immutable).
+	 */
+	public static final HttpHeaders EMPTY = new HttpHeaders(new LinkedHashMap<>(0), true);
 	/**
 	 * The HTTP {@code Accept} header field name.
 	 * @see <a href="http://tools.ietf.org/html/rfc7231#section-5.3.2">Section 5.3.2 of RFC 7231</a>