Update ServletPortletHelper.java (#12087)

This is needed to get /UIDL working behind a reverse proxy. Without the change pathInfo would be "/UIDL" but prefix would be "/UIDL/" in line 105.

server/src/main/java/com/vaadin/server/ServletPortletHelper.java

             prefix = '/' + prefix;
         }
 
+        if (!pathInfo.endsWith("/") && prefix.endsWith("/")) {
+            pathInfo += '/';
+        }
+
         if (pathInfo.startsWith(prefix)) {
             return true;
         }