Added a convenience method.

This change is originally from pull request #682.

core/src/main/java/hudson/Util.java

@@ -513,6 +513,25 @@ public class Util {
         else                return s;
     }
 
+
+    /**
+     * Ensure string ends with suffix
+     *
+     * @param subject Examined string
+     * @param suffix  Desired suffix
+     * @return Original subject in case it already ends with suffix, null in
+     *         case subject was null and subject + suffix otherwise.
+     * @since 1.505
+     */
+    public static String ensureEndsWith(String subject, String suffix) {
+
+        if (subject == null) return null;
+
+        if (subject.endsWith(suffix)) return subject;
+
+        return subject + suffix;
+    }
+
     /**
      * Computes MD5 digest of the given input stream.
      *

core/src/main/java/jenkins/model/Jenkins.java

@@ -1868,8 +1868,7 @@ public class Jenkins extends AbstractCIBase implements ModifiableTopLevelItemGro
     public String getRootUrl() {
         String url = JenkinsLocationConfiguration.get().getUrl();
         if(url!=null) {
-            if (!url.endsWith("/")) url += '/';
-            return url;
+            return Util.ensureEndsWith(url,"/");
         }
         StaplerRequest req = Stapler.getCurrentRequest();
         if(req!=null)