Consume getProtocolResolvers() rather than internal list

This commit makes sure that DefaultResourceLoader consistently use
getProtocolResolvers() to access additional protocol resolvers. This
allows subclasses to define how the list is provided.

Closes gh-23564

spring-core/src/main/java/org/springframework/core/io/DefaultResourceLoader.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -144,7 +144,7 @@ public class DefaultResourceLoader implements ResourceLoader {
 	public Resource getResource(String location) {
 		Assert.notNull(location, "Location must not be null");
 
-		for (ProtocolResolver protocolResolver : this.protocolResolvers) {
+		for (ProtocolResolver protocolResolver : getProtocolResolvers()) {
 			Resource resource = protocolResolver.resolve(location, this);
 			if (resource != null) {
 				return resource;