PathPattern does not use custom separator

Prior to this commit, `PathPattern::extractPathWithinMapping`
would always use the default path pattern separator `/` when extracting
the path within the pattern of a matched route.

This commit ensures that `PathPattern` uses the configured separator
when extracting the path within the matched mapping.

Fixes gh-23168

spring-web/src/main/java/org/springframework/web/util/pattern/PathPattern.java

@@ -339,7 +339,7 @@ public class PathPattern implements Comparable<PathPattern> {
 					}
 				}
 			}
-			resultPath = PathContainer.parsePath(buf.toString());
+			resultPath = PathContainer.parsePath(buf.toString(), String.valueOf(this.separator));
 		}
 		else if (startIndex >= endIndex) {
 			resultPath = PathContainer.parsePath("");