Fix potential Jetty MalformedURLException

Update JettyEmbeddedServletContainerFactory to use the JarResource
class to create the base resource. This prevents a potential
"java.lang.NullPointerException: no !/ in spec" error which can occur
when using shaded executable jars.

Fixes gh-2245

Author: cemo

spring-boot/src/main/java/org/springframework/boot/context/embedded/jetty/JettyEmbeddedServletContainerFactory.java

@@ -40,6 +40,7 @@
 import org.eclipse.jetty.servlet.ErrorPageErrorHandler;
 import org.eclipse.jetty.servlet.ServletHolder;
 import org.eclipse.jetty.servlet.ServletMapping;
+import org.eclipse.jetty.util.resource.JarResource;
 import org.eclipse.jetty.util.resource.Resource;
 import org.eclipse.jetty.util.ssl.SslContextFactory;
 import org.eclipse.jetty.webapp.AbstractConfiguration;
@@ -259,7 +260,8 @@ private void configureDocumentRoot(WebAppContext handler) {
 		if (root != null) {
 			try {
 				if (!root.isDirectory()) {
-					Resource resource = Resource.newResource("jar:" + root.toURI() + "!");
+					Resource resource = JarResource.newJarResource(Resource
+							.newResource(root));
 					handler.setBaseResource(resource);
 				}
 				else {