### Description

I want modify `Accept` header using include_hf_value. Need to add additional values to current. I do this using. ``` include_hf_value("Accept", "application/sdp"); include_hf_value("Accept", "application/pidf+xml"); ``` Expected result in SIP message ``` Accept: application/sdp, application/pidf+xml ``` Realy i get ``` Accept: application/sdp Accept: application/pidf+xml ```

### Troubleshooting

#### Reproduction Need to add this to kamailio config ``` Accept: application/sdp Accept: application/pidf+xml ```

#### Debugging Data None

#### Log Messages None

#### SIP Traffic

PCAP file is attached.

### Possible Solutions None

### Additional Information Used kamailio 5.1 branch commit 9f04def504051dc8da3989b1f96e4b6633d31a75 Docker container kamailio/kamailio-ci ``` / # kamailio -v version: kamailio 5.1.5 (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 on 10:41:20 Aug 23 2018 with gcc 6.4.0 ```

* **Operating System**:

``` / # cat /etc/os-release NAME="Alpine Linux" ID=alpine VERSION_ID=3.8.0 PRETTY_NAME="Alpine Linux v3.8" HOME_URL="http://alpinelinux.org" BUG_REPORT_URL="http://bugs.alpinelinux.org" / # uname -a Linux 088bedf3b82f 4.14.13-300.fc27.x86_64 #1 SMP Thu Jan 11 04:00:01 UTC 2018 x86_64 Linux ```