Proactively reject URLs without target address

Issue: SPR-15782

spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpConnector.java

@@ -68,6 +68,10 @@ public class ReactorClientHttpConnector implements ClientHttpConnector {
 	public Mono<ClientHttpResponse> connect(HttpMethod method, URI uri,
 			Function<? super ClientHttpRequest, Mono<Void>> requestCallback) {
 
+		if (!uri.isAbsolute()) {
+			return Mono.error(new IllegalArgumentException("URI is not absolute: " + uri));
+		}
+
 		return this.httpClient
 				.request(adaptHttpMethod(method),
 						uri.toString(),