Add leading slash for path predicate if not present

This commit adds a leading slash for path predicates in both
WebFlux.fn and WebMvc.fn.

Closes gh-22795

spring-webmvc/src/main/java/org/springframework/web/servlet/function/RequestPredicates.java

@@ -107,6 +107,9 @@ public abstract class RequestPredicates {
 	 */
 	public static RequestPredicate path(String pattern) {
 		Assert.notNull(pattern, "'pattern' must not be null");
+		if (!pattern.isEmpty() && !pattern.startsWith("/")) {
+			pattern = "/" + pattern;
+		}
 		return pathPredicates(DEFAULT_PATTERN_PARSER).apply(pattern);
 	}