Consistent ordering in MethodNotAllowedException

This caused random failures in a newly added test. Also remove defensive
check in ResponseStatusExceptionResolver.

See gh-24944

spring-web/src/main/java/org/springframework/web/server/MethodNotAllowedException.java

@@ -18,7 +18,7 @@
 
 import java.util.Collection;
 import java.util.Collections;
-import java.util.HashSet;
+import java.util.LinkedHashSet;
 import java.util.Map;
 import java.util.Set;
 
@@ -54,7 +54,7 @@ public MethodNotAllowedException(String method, @Nullable Collection<HttpMethod>
 			supportedMethods = Collections.emptySet();
 		}
 		this.method = method;
-		this.httpMethods = Collections.unmodifiableSet(new HashSet<>(supportedMethods));
+		this.httpMethods = Collections.unmodifiableSet(new LinkedHashSet<>(supportedMethods));
 	}
 
 

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/annotation/ResponseStatusExceptionResolver.java

@@ -130,11 +130,8 @@ protected ModelAndView resolveResponseStatus(ResponseStatus responseStatus, Http
 	protected ModelAndView resolveResponseStatusException(ResponseStatusException ex,
 			HttpServletRequest request, HttpServletResponse response, @Nullable Object handler) throws Exception {
 
-		ex.getResponseHeaders().forEach((name, values) -> {
-			if (response.getHeader(name) == null) {
-				values.forEach(value -> response.addHeader(name, value));
-			}
-		});
+		ex.getResponseHeaders().forEach((name, values) ->
+				values.forEach(value -> response.addHeader(name, value)));
 
 		int statusCode = ex.getStatus().value();
 		String reason = ex.getReason();