Honor ProtocolResolvers in GenericApplicationContext

When the ProtocolResolver SPI was introduced in Spring Framework 4.3,
support for protocol resolvers was added in DefaultResourceLoader's
getResource() implementation; however, GenericApplicationContext's
overridden getResource() implementation was not updated accordingly.

Prior to this commit, if a GenericApplicationContext was configured
with a custom ResourceLoader, registered protocol resolvers were
ignored.

This commit ensures that protocol resolvers are honored in
GenericApplicationContext even if a custom ResourceLoader is used.

Closes gh-28703

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2022 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.
@@ -32,7 +32,8 @@ import org.springframework.util.StringUtils;
 
 /**
  * Default implementation of the {@link ResourceLoader} interface.
- * Used by {@link ResourceEditor}, and serves as base class for
+ *
+ * <p>Used by {@link ResourceEditor}, and serves as base class for
  * {@link org.springframework.context.support.AbstractApplicationContext}.
  * Can also be used standalone.
  *
@@ -114,6 +115,7 @@ public class DefaultResourceLoader implements ResourceLoader {
 	 * Return the collection of currently registered protocol resolvers,
 	 * allowing for introspection as well as modification.
 	 * @since 4.3
+	 * @see #addProtocolResolver(ProtocolResolver)
 	 */
 	public Collection<ProtocolResolver> getProtocolResolvers() {
 		return this.protocolResolvers;