safari: dont remove a=extmap-allow-mixed in current safari

which does support parsing the extmap-allow-mixed attribute. See
  https://bugs.chromium.org/p/webrtc/issues/detail?id=9985

src/js/common_shim.js

@@ -318,19 +318,22 @@ export function shimConnectionState(window) {
 }
 
 export function removeAllowExtmapMixed(window) {
-  /* remove a=extmap-allow-mixed for Chrome < M71 */
+  /* remove a=extmap-allow-mixed for webrtc.org < M71 */
   if (!window.RTCPeerConnection) {
     return;
   }
   const browserDetails = utils.detectBrowser(window);
   if (browserDetails.browser === 'chrome' && browserDetails.version >= 71) {
     return;
   }
+  if (browserDetails.browser === 'safari' && browserDetails.version >= 605) {
+    return;
+  }
   const nativeSRD = window.RTCPeerConnection.prototype.setRemoteDescription;
   window.RTCPeerConnection.prototype.setRemoteDescription =
   function setRemoteDescription(desc) {
     if (desc && desc.sdp && desc.sdp.indexOf('\na=extmap-allow-mixed') !== -1) {
-      desc.sdp = desc.sdp.split('\n').filter((line) => {
+      arguments[0] = {type: desc.type, sdp: desc.sdp.split('\n').filter((line) => {
         return line.trim() !== 'a=extmap-allow-mixed';
       }).join('\n');
     }