add sip_copy_multipart to work like sip_copy_custom_headers

src/mod/endpoints/mod_sofia/mod_sofia.c

@@ -3845,6 +3845,10 @@ static switch_call_cause_t sofia_outgoing_channel(switch_core_session_t *session
 		if (!(vval = switch_channel_get_variable(o_channel, "sip_copy_custom_headers")) || switch_true(vval)) {
 			switch_ivr_transfer_variable(session, nsession, SOFIA_SIP_HEADER_PREFIX_T);
 		}
+
+		if (!(vval = switch_channel_get_variable(o_channel, "sip_copy_multipart")) || switch_true(vval)) {
+			switch_ivr_transfer_variable(session, nsession, SOFIA_MULTIPART_PREFIX_T);
+		}
 		switch_ivr_transfer_variable(session, nsession, "sip_video_fmtp");
 		switch_ivr_transfer_variable(session, nsession, "sip-force-contact");
 		switch_ivr_transfer_variable(session, nsession, "sip_sticky_contact");

src/mod/endpoints/mod_sofia/mod_sofia.h

@@ -89,6 +89,7 @@ typedef struct private_object private_object_t;
 #define SOFIA_USER_AGENT "FreeSWITCH-mod_sofia/" SWITCH_VERSION_MAJOR "." SWITCH_VERSION_MINOR "." SWITCH_VERSION_MICRO "-" SWITCH_VERSION_REVISION
 #define SOFIA_CHAT_PROTO "sip"
 #define SOFIA_MULTIPART_PREFIX "sip_mp_"
+#define SOFIA_MULTIPART_PREFIX_T "~sip_mp_"
 #define SOFIA_SIP_HEADER_PREFIX "sip_h_"
 #define SOFIA_SIP_RESPONSE_HEADER_PREFIX "sip_rh_"
 #define SOFIA_SIP_BYE_HEADER_PREFIX "sip_bye_h_"