compression: Do not strip "a=rtpmap" lines with types 96-97

According to the IANA RTP payload types register, the dynamic payload
types (custom-defined, for which the "a=rtpmap" lines are mandatory and
cannot be stripped) begin at 96, not at 98.

Reported by JP Hindin

modules/compression/compression.c

@@ -587,7 +587,7 @@ static mc_whitelist_p mc_dup_whitelist(mc_whitelist_p src)
  * 2) Header names with compact forms will be transformed to
  * compact form
  * 3) Headers which not in whitelist will be removed
- * 4) Unnecessary sdp body codec attributes lower than 98 removed
+ * 4) Unnecessary sdp body codec attributes lower than 96 removed
  */
 static int mc_compact(struct sip_msg* msg, mc_whitelist_p wh_list)
 {
@@ -751,7 +751,7 @@ static int mc_compact_cb(char** buf_p, mc_whitelist_p wh_list, int type, int* ol
 				(buf_cpy++, rtpmap_len++);
 			}
 
-			if (rtpmap_val < 98) {
+			if (rtpmap_val < 96) {
 				msg_total_len += frg->end - frg->begin + 1;
 				frg->next = pkg_malloc(sizeof(body_frag_t));
 				if (!frg->next)

modules/compression/doc/compression_admin.xml

@@ -229,7 +229,7 @@ xlog("compression registered\n");
 		and are not in the whitelist will be removed, headers of same type will be merged
 		together, separated by ',', header names which have a short form
 		will be reduced to that short form and SDP rtpmap attribute headers which contain
-		a value lower than 98 will be removed, because they are not mandatory. Lumps are not
+		a value lower than 96 will be removed, because they are not mandatory. Lumps are not
 		affected by this function, because it is applied after all messages changes are
 		processed.
 		done.