Ensure getResources("") includes nested root URLs

Previously, if Boot's JarURLConnection pointed to the root of a nested
entry, e.g. /BOOT-INF/classes, a call to getInputStream() would throw
an IOException. This behavior is reasonable for a URL that points
to the root of a normal jar as the jar itself is on the class path
anyway. However, for a nested jar it meant that a call to
ClassLoader.getResources("") would not include URLs for any nested
jars and directories (/BOOT-INF/classes and jars in /BOOT-INF/lib).
This is due to some logic in URLClassPath.Loader.findResource that
verifies a URL by opening a connection and calling getInputStream().

The result of missing URLs for the root of nested jars and directories
is that classpath scanning that scans from the root (not a good idea
for performance reasons, but something that we should support) would
not find entries in /BOOT-INF/classes or in jars in /BOOT-INF/lib.

This commit updates our JarURLConnection so that it no longer throws
an IOException when asked for an InputStream for the root of a nested
entry (directory or jar).

Fixes gh-7003

spring-boot-tools/spring-boot-loader/src/test/java/org/springframework/boot/loader/jar/JarFileTests.java

@@ -28,6 +28,7 @@ import java.net.URLClassLoader;
 import java.nio.charset.Charset;
 import java.util.Enumeration;
 import java.util.jar.JarEntry;
+import java.util.jar.JarInputStream;
 import java.util.jar.Manifest;
 import java.util.zip.ZipEntry;
 
@@ -54,6 +55,7 @@ import static org.mockito.Mockito.verify;
  * @author Andy Wilkinson
  */
 public class JarFileTests {
+
 	private static final String PROTOCOL_HANDLER = "java.protocol.handler.pkgs";
 
 	private static final String HANDLERS_PACKAGE = "org.springframework.boot.loader";
@@ -270,6 +272,12 @@ public class JarFileTests {
 		assertThat(conn.getJarFile()).isSameAs(nestedJarFile);
 		assertThat(conn.getJarFileURL().toString())
 				.isEqualTo("jar:" + this.rootJarFile.toURI() + "!/nested.jar");
+		assertThat(conn.getInputStream()).isNotNull();
+		JarInputStream jarInputStream = new JarInputStream(conn.getInputStream());
+		assertThat(jarInputStream.getNextJarEntry().getName()).isEqualTo("3.dat");
+		assertThat(jarInputStream.getNextJarEntry().getName()).isEqualTo("4.dat");
+		assertThat(jarInputStream.getNextJarEntry().getName()).isEqualTo("\u00E4.dat");
+		jarInputStream.close();
 		assertThat(conn.getPermission()).isInstanceOf(FilePermission.class);
 		FilePermission permission = (FilePermission) conn.getPermission();
 		assertThat(permission.getActions()).isEqualTo("read");