Update GrailsWebRequest

to read the PORT information from the request header when it is missing
in the request. This fixes #12816 where application is accesed via
AWS's ALB.

grails-web-common/src/main/groovy/org/grails/web/servlet/mvc/GrailsWebRequest.java

@@ -466,7 +466,7 @@ public String getBaseUrl() {
             StringBuilder sb=new StringBuilder();
             sb.append(scheme).append("://").append(request.getServerName());
 
-            int port = request.getServerPort();
+            int port = request.getServerPort() ?: request.getHeader("X-Forwarded-Port");
 
             //ignore port append if the request was forwarded from a VIP as actual source port is now not known
             if (forwardedScheme == null && (("http".equals(scheme) && port != 80) || ("https".equals(scheme) && port != 443))) {