IOS WEBRTC RTCPeerConnection必须默认设置DtlsSrtpKeyAgreement，否则请求失败

http://www./docs/WebRTC-PeerConnection.html

var STUN = {
    urls: 'stun:stun.l.google.com:19302'
};

var TURN = {
    urls: 'turn:turn.bistri.com:80',
    credential: 'homeo',
    username: 'homeo'
};

var iceServers = {
   iceServers: [STUN, TURN]
};

// DTLS/SRTP is preferred on chrome
// to interop with Firefox
// which supports them by default

var DtlsSrtpKeyAgreement = {
   DtlsSrtpKeyAgreement: true
};

var optional = {
   optional: [DtlsSrtpKeyAgreement]
};

 var peer = new [webkit|moz]RTCPeerConnection(iceServers, optional);

   peer.addStream(mediaStream);
        
        peer.onaddstream = function(mediaStream) {
            video.src = webkitURL.createObjectURL(mediaStream);
        };

        peer.onicecandidate = function(event) {
            var candidate = event.candidate;
            if(candidate) {
                socket.send({
                    targetUser: 'target-user-id',
                    candidate: candidate
                });
            }
        };
        
        // remote-descriptions should be set earlier
        // using offer-sdp provided by the offerer
        var remoteDescription = new RTCSessionDescription(offerSDP);
        peer.setRemoteDescription(remoteDescription, successCallback, failureCallback);
		
        peer.createAnswer(function(answerSDP) {
            peer.setLocalDescription(answerSDP, successCallback, failureCallback);
            socket.send({
                targetUser: 'target-user-id',
                answerSDP: answerSDP
            });
        }, onfailure, sdpConstraints);
    }