Don't require MID RTP header extension.

Some implementations (e.g. gstreamer) seems not to propose MID RTP
header extension in their SDP. While this is quite uncommon, they can
still demux incoming traffic based on ssrc (if included in SDP by the sender)
or payload type.

This commit allows to send data without MID RTP header extension being
negotiated. To be consistent, we also don't require users of Elixir
WebRTC to use MID header extension in their SDP.

Author: mickel8

lib/ex_webrtc/peer_connection/configuration.ex

@@ -224,7 +224,11 @@ defmodule ExWebRTC.PeerConnection.Configuration do
     extensions = Keyword.get(options, :rtp_header_extensions, @default_rtp_header_extensions)
 
     unless %{type: :all, uri: @mid_uri} in extensions do
-      raise "Mandatory MID RTP header extensions was not found in #{inspect(extensions)}"
+      Logger.warning("""
+      MID RTP header extension was not found in #{inspect(extensions)}.
+      While this is correct, it is strongly recommended to include \
+      MID RTP header extension to avoid any difficulties with packet demultiplexing.
+      """)
     end
 
     {audio_extensions, video_extensions} =

lib/ex_webrtc/rtp_sender.ex

@@ -134,23 +134,32 @@ defmodule ExWebRTC.RTPSender do
   @doc false
   @spec send_packet(sender(), ExRTP.Packet.t(), boolean()) :: {binary(), sender()}
   def send_packet(sender, packet, rtx?) do
-    %Extmap{} = mid_extmap = Map.fetch!(sender.rtp_hdr_exts, @mid_uri)
-
-    mid_ext =
-      %ExRTP.Packet.Extension.SourceDescription{text: sender.mid}
-      |> ExRTP.Packet.Extension.SourceDescription.to_raw(mid_extmap.id)
-
     {pt, ssrc} =
       if rtx? do
         {sender.rtx_pt, sender.rtx_ssrc}
       else
         {sender.pt, sender.ssrc}
       end
 
+    packet = %{packet | payload_type: pt, ssrc: ssrc}
+
+    # Add mid header extension only if it was negotiated.
+    # The receiver can still demux packets based
+    # on ssrc (if it was included in sdp) or payload type.
     packet =
-      %{packet | payload_type: pt, ssrc: ssrc}
-      |> ExRTP.Packet.remove_extension(mid_extmap.id)
-      |> ExRTP.Packet.add_extension(mid_ext)
+      case Map.get(sender.rtp_hdr_exts, @mid_uri) do
+        %Extmap{} = mid_extmap ->
+          mid_ext =
+            %ExRTP.Packet.Extension.SourceDescription{text: sender.mid}
+            |> ExRTP.Packet.Extension.SourceDescription.to_raw(mid_extmap.id)
+
+          packet
+          |> ExRTP.Packet.remove_extension(mid_extmap.id)
+          |> ExRTP.Packet.add_extension(mid_ext)
+
+        nil ->
+          packet
+      end
 
     report_recorder =
       if sender.reports? do