ensure root URL ends with slash

[jetersen]

the getRootUrl() behaves differently, however it ensures that JenkinsLocationConfiguration.get().getUrl() ends with /

    public String getRootUrl() throws IllegalStateException {
        JenkinsLocationConfiguration config = JenkinsLocationConfiguration.get();
        if (config == null) {
            Jenkins j = getInstance();
            throw new IllegalStateException("Jenkins instance " + j + " has been successfully initialized, but JenkinsLocationConfiguration is undefined.");
        } else {
            String url = config.getUrl();
            if (url != null) {
                return Util.ensureEndsWith(url, "/");
            } else {
                StaplerRequest req = Stapler.getCurrentRequest();
                return req != null ? this.getRootUrlFromRequest() : null;
            }
        }
    }

[jetersen]

@ziouf please take a look :)