WebRTC.MediaHandler#addStreams: work if passed single stream
WebRTC.MediaHandler#addStreams: work if passed single stream

This allows backward compatibility with custom MediaStreamManager
implementations that return a MediaStream instead of an array thereof.

markandrus/SIP.js,Escaux/SIP.js,onsip/SIP.js,onsip/SIP.js,josephfrazier/SIP.js,joseph-onsip/SIP.js,ronlobo/SIP.js