### Description

Testing on latest master deb nightly build (`5.3.0~dev7+0~20190904005559.1479+buster`) I've noticed that `$Au` is not being set correctly, at least according to my tests.

From the documentation (https://www.kamailio.org/wiki/cookbooks/devel/pseudovariables#au_-_acc_usern...):

_$Au - Acc username: username for accounting purposes. It's a selective pseudo variable (inherited from acc module). It returns auth username ($au) if exists or From username ($fU) otherwise._

I have the following log statement in several places in a test box:

xlog("L_NOTICE", "[DEBUG] Au=$Au au=$au aU=$aU ar=$ar fU=$fU fu=$fu - \n");

And when I run through them I get the following:

Initial log (at the beginning of request_route):

``` Sep 5 01:54:39 csbc01 csbc[20380]: NOTICE: {1 1 REGISTER f597522fd7e52fe99f2a54a834101efa@0:0:0:0:0:0:0:0} <script>: [DEBUG] Au=1063362@some.domain au=<null> aU=<null> ar=<null> fU=1063362 fu=sip:1063362@some.domain - ```

After a 401 challenge, the following log statements look like this one:

``` Sep 5 01:54:40 csbc01 csbc[20385]: NOTICE: {1 2 REGISTER f597522fd7e52fe99f2a54a834101efa@0:0:0:0:0:0:0:0} <script>: [DEBUG] Au=1063362@some.domain au=1063362 aU=1063362 ar=custom-realm fU=1063362 fu=sip:1063362@some.domain - ```

At this point I would expect `$Au` to not have the `@domain` part in it.

And strangely, after an `auth_check()`, it changes again:

``` Sep 5 01:54:40 csbc01 csbc[20385]: NOTICE: {1 2 REGISTER f597522fd7e52fe99f2a54a834101efa@0:0:0:0:0:0:0:0} <script>: [DEBUG] Au=1063362@custom-realm au=1063362 aU=1063362 ar=custom-realm fU=1063362 fu=sip:1063362@some.domain - ```

From that point onward, it no longer has the domain and now contains the realm.

I believe both these behaviors are incorrect, or at least they don't match what the documentation says.

### Troubleshooting

@henningw commented about this in the email thread I started:

_Hello Joel,_

_I did a quick look into the code, this is basically unchanged since 2008. I also tested 5.2.3, it shows the same behavior._

_NOTICE: {1 1 REGISTER 363545605-1445114084-1880652839} <script>: [DEBUG] Au=82621@116.203.XXX.XXX au=<null> aU=<null> fU=82621 fu=sip:82621@116.203.XXX.XXX - root@sip:~# kamcmd core.version kamailio 5.2.3 (x86_64/linux) I certainly would make more sense in a way like it is documented. Can you easily check an older version?_

_Cheers,_

_Henning_

### Reproduction

Print `$Au` in logs before challenge, after a successful auth, and after an `auth_check()`

### Additional Information

* **Kamailio Version** - output of `kamailio -v`

``` root@csbc01:/etc/kamailio# kamailio -v version: kamailio 5.3.0-dev7 (x86_64/linux) flags: USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MMAP, PKG_MALLOC, Q_MALLOC, F_MALLOC, TLSF_MALLOC, DBG_SR_MEMORY, USE_FUTEX, FAST_LOCK-ADAPTIVE_WAIT, USE_DNS_CACHE, USE_DNS_FAILOVER, USE_NAPTR, USE_DST_BLACKLIST, HAVE_RESOLV_RES ADAPTIVE_WAIT_LOOPS 1024, MAX_RECV_BUFFER_SIZE 262144, MAX_URI_SIZE 1024, BUF_SIZE 65535, DEFAULT PKG_SIZE 8MB poll method support: poll, epoll_lt, epoll_et, sigio_rt, select. id: unknown compiled with gcc 8.3.0 root@csbc01:/etc/kamailio# ```

* **Operating System**:

<!-- Details about the operating system, the type: Linux (e.g.,: Debian 8.4, Ubuntu 16.04, CentOS 7.1, ...), MacOS, xBSD, Solaris, ...; Kernel details (output of `uname -a`) -->

``` Distributor ID: Debian Description: Debian GNU/Linux 10 (buster) Release: 10 Codename: buster ```