Session.js: InviteServerContext - add receiveNonInviteResponse

This allows the InviteServerContext to be able to send in-dialog MESSAGE
requests. See
https://groups.google.com/forum/#!msg/sip_js/H27XRsTnBhA/-XaGf_bbjJAJ
WebRTC/MediaHandler.js: make prototype properties writable

This makes it easier to create custom MediaHandlers by modifying the default
one.