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-webmvc/src/main/java/org/springframework/web/servlet/mvc/condition/RequestMethodsRequestCondition.java

@@ -157,9 +157,8 @@ public final class RequestMethodsRequestCondition extends AbstractRequestConditi
 
 	@Nullable
 	private RequestMethodsRequestCondition matchRequestMethod(String httpMethodValue) {
-		RequestMethod requestMethod;
-		try {
-			requestMethod = RequestMethod.valueOf(httpMethodValue);
+		RequestMethod requestMethod = RequestMethod.resolve(httpMethodValue);
+		if (requestMethod != null) {
 			if (getMethods().contains(requestMethod)) {
 				return requestMethodConditionCache.get(httpMethodValue);
 			}
@@ -167,9 +166,6 @@ public final class RequestMethodsRequestCondition extends AbstractRequestConditi
 				return requestMethodConditionCache.get(HttpMethod.GET.name());
 			}
 		}
-		catch (IllegalArgumentException ex) {
-			// Custom request method
-		}
 		return null;
 	}