Fix regression in WebFlux support for WebDAV methods

This commit ensures that WebFlux's RequestMethodsRequestCondition
supports HTTP methods that are not in the RequestMethod enum.

- RequestMethod::resolve is introduced, to convert from a HttpMethod
(name) to enum values.
- RequestMethod::asHttpMethod is introduced, to convert from enum value
to HttpMethod.
- HttpMethod::valueOf replaced Map-based lookup to a switch statement
- Enabled tests that check for WebDAV methods

See gh-27697
Closes gh-29981

spring-web/src/test/java/org/springframework/web/bind/annotation/RequestMethodTests.java

@@ -0,0 +1,59 @@
+/*
+ * Copyright 2002-2023 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.web.bind.annotation;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.HttpMethod;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author Arjen Poutsma
+ */
+class RequestMethodTests {
+
+	@Test
+	void resolveString() {
+		String[] methods = new String[]{"GET", "HEAD", "POST", "PUT", "PATCH", "DELETE", "OPTIONS", "TRACE"};
+		for (String httpMethod : methods) {
+			RequestMethod requestMethod = RequestMethod.resolve(httpMethod);
+			assertThat(requestMethod).isNotNull();
+			assertThat(requestMethod.name()).isEqualTo(httpMethod);
+		}
+		assertThat(RequestMethod.resolve("PROPFIND")).isNull();
+	}
+
+	@Test
+	void resolveHttpMethod() {
+		for (HttpMethod httpMethod : HttpMethod.values()) {
+			RequestMethod requestMethod = RequestMethod.resolve(httpMethod);
+			assertThat(requestMethod).isNotNull();
+			assertThat(requestMethod.name()).isEqualTo(httpMethod.name());
+		}
+		assertThat(RequestMethod.resolve(HttpMethod.valueOf("PROPFIND"))).isNull();
+	}
+
+	@Test
+	void asHttpMethod() {
+		for (RequestMethod requestMethod : RequestMethod.values()) {
+			HttpMethod httpMethod = requestMethod.asHttpMethod();
+			assertThat(httpMethod).isNotNull();
+			assertThat(httpMethod.name()).isEqualTo(requestMethod.name());
+		}
+	}
+}