Bug#37309796 - Inconsistent authentication parameter format and names for MRS Authentication [POSTFIX]

Description
===========
New code had disabled old clients, adding support for new and old clients.

(POST-PUSH FIX WL#15440)

Change-Id: Ia8620bec7aff93f59647c914536b504b5496240c

Author: lkotula

router/src/mysql_rest_service/src/mrs/authentication/helper/scram.cc

@@ -191,7 +191,9 @@ class JsonAuthInitRequest
  public:
   bool String(const Ch *cstr, rapidjson::SizeType clength, bool) override {
     if (!is_object_path()) return true;
-    if ("username" == get_current_key())
+    if ("user" == get_current_key() && result_.user.empty())
+      result_.user.assign(cstr, clength);
+    else if ("username" == get_current_key())
       result_.user.assign(cstr, clength);
     else if ("nonce" == get_current_key())
       result_.nonce.assign(cstr, clength);