CORS support in HTTP method predicate

This commit introduces CORS support for the HttpMethodPredicate in
WebMvc.fn and WebFlux.fn.

Closes gh-24564

spring-webflux/src/main/java/org/springframework/web/reactive/function/server/RequestPredicates.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -52,6 +52,7 @@ import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.MultiValueMap;
+import org.springframework.web.cors.reactive.CorsUtils;
 import org.springframework.web.reactive.function.BodyExtractor;
 import org.springframework.web.server.ServerWebExchange;
 import org.springframework.web.server.WebSession;
@@ -449,11 +450,25 @@ public abstract class RequestPredicates {
 
 		@Override
 		public boolean test(ServerRequest request) {
-			boolean match = this.httpMethods.contains(request.method());
-			traceMatch("Method", this.httpMethods, request.method(), match);
+			HttpMethod method = method(request);
+			boolean match = this.httpMethods.contains(method);
+			traceMatch("Method", this.httpMethods, method, match);
 			return match;
 		}
 
+		@Nullable
+		private static HttpMethod method(ServerRequest request) {
+			if (CorsUtils.isPreFlightRequest(request.exchange().getRequest())) {
+				String accessControlRequestMethod =
+						request.headers().firstHeader(HttpHeaders.ACCESS_CONTROL_REQUEST_METHOD);
+				return HttpMethod.resolve(accessControlRequestMethod);
+			}
+			else {
+				return request.method();
+			}
+		}
+
+
 		@Override
 		public void accept(Visitor visitor) {
 			visitor.method(Collections.unmodifiableSet(this.httpMethods));