Combined class and method @RequestMapping without patterns should match root path as both "" and "/"

[rstoyanchev]

Trailing slash matching was deprecated for 6.0 and the default changed to false. The motivation, as discussed in #28552, is to make mappings transparent by eliminating extras such as this, as well as others deprecated previously such as suffix patterns, path segment trimming, path decoding, and others that create a potential for vulnerabilities.

While the recommendation remains to configure redirects where trailing slashes need to be supported instead of relying on such an application-wide request mapping feature, an @RequestMapping without patterns is arguably one place where an exception can be made, and could be seen less as a trailing slash match and more as the root URL. For example:

@RestController
public class HomeController {

    @GetMapping
    public String home() {
        return "Hello, World!";
    }

}

The above should match to both http://localhost:8080 and to http://localhost:8080/. In both cases the requestURI is /, which means that the above would otherwise never match. There are other cases where the lookup path may be "", e.g. where there is a contextPath and/or servletPath, but arguably in all of these cases it's unlikely to have any ambiguity with URL security, which should be securing the root URL of the application consistently.