Add method to get the defaultServlet MappedResource from ServletHandler

[lachlan-roberts]

This is needed to improve the way we find the servlet mapped to the default mapping in appengine:
https://github.com/GoogleCloudPlatform/appengine-java-standard/blob/97f644a3b5a3edfccb06e58f7401e047b6d28c48/runtime/runtime_impl_jetty12/src/main/java/com/google/apphosting/runtime/jetty/ee10/ResourceFileServlet.java#L259

Currently it does handler.getMappedServlet("/") which will actually return the servlet from a context root mapping so is incorrect.

[gregw]

Not sure I understand the need?

[gregw]

@lachlan-roberts I think this is the logic you need

@Test
public void testXServletMappings() throws Exception
{
    Server server = new Server();
    ServletContextHandler context = new ServletContextHandler("/");
    server.setHandler(context);
    ServletHandler handler = new ServletHandler();
    context.setHandler(handler);

    handler.addServletWithMapping(SomeServlet.class.getName(), "/path");
    handler.addServletWithMapping(DefaultServlet.class.getName(), "/");
    handler.addServletWithMapping(SomeServlet.class.getName(), "");

    server.start();

    ServletHandler.MappedServlet mapped = handler.getMappedServlet("/something");
    assertEquals(mapped.getServletHolder().getName(), handler.getServletMapping("/").getServletName());

    mapped = handler.getMappedServlet("/");
    assertNotEquals(mapped.getServletHolder().getName(), handler.getServletMapping("/").getServletName());

    mapped = handler.getMappedServlet("/path");
    assertNotEquals(mapped.getServletHolder().getName(), handler.getServletMapping("/").getServletName()); 
}

[lachlan-roberts]

Closing as we can use getServletMapping() which compares on pathSpec.