### Description

If var is encoded with `base58`, it can't be restored via `s.decode.base58`

#### Reproduction ``` request_route { route(TEST_BASE58); }

route[TEST_BASE58] { $var(test_var) = "This is a text"; $var(test_var_encode_base58) = $(var(test_var){s.encode.base58}); $var(test_var_decode_base58) = $(var(test_var_encode_base58){s.decode.base58}); xlog("L_ALERT", "[BASE58 TEST] $var(test_var) -> $var(test_var_encode_base58) -> $var(test_var_decode_base58) \n");

}

``` #### Log Messages

<!-- Check the syslog file and if there are relevant log messages printed by Kamailio, add them next, or attach to issue, or provide a link to download them (e.g., to a pastebin site). -->

2 messages just to show, that var is not restored ``` 44(60) ALERT: <script>: [BASE58 TEST] This is a text -> Y2oxCuAXwVJduYPTnFd -> 14(30) ALERT: <script>: [BASE58 TEST] This is a text -> Y2oxCuAXwVJduYPTnFd -> ```

### Additional Information

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

``` version: kamailio 5.2.0 (x86_64/linux) flags: STATS: Off, USE_TCP, USE_TLS, USE_SCTP, TLS_HOOKS, USE_RAW_SOCKS, DISABLE_NAGLE, USE_MCAST, DNS_IP_HACK, SHM_MEM, 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 5.3.1 ```

* **Operating System**:

``` 4.4.0-135-generic #161-Ubuntu SMP Mon Aug 27 10:45:01 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux ```