Properly handle unknown media-types in INFO

Send a 415 Unsupported Media Type response if an INFO message contains an unknown media-type.

I noticed this during interop with FreeSwitch. FreeSwitch sends an INFO message:

    INFO sip:trnrph3g@8ovnkn8jm6fs.invalid;transport=ws;ob SIP/2.0
    Via: SIP/2.0/WS 127.0.0.1:8060;branch=z9hG4bKe5S382rp2N4ta
    Max-Forwards: 70
    From: <sip:conf@127.0.0.1;transport=ws>;tag=jFvKFSK4pHQ2F
    To: <sip:alice@example.com>;tag=rk0p55l0lt
    Call-ID: 9tkif8fahbk1guksim84
    CSeq: 56066404 INFO
    Contact: <sip:conf@127.0.0.1:5060;transport=udp>
    User-Agent: websocket server
    Allow: INVITE, ACK, BYE, CANCEL, OPTIONS, MESSAGE, INFO, UPDATE, REGISTER, NOTIFY
    Supported: timer, precondition, path, replaces
    Content-Type: application/media_control+xml
    Content-Length: 175
   
    <?xml version="1.0" encoding="utf-8" ?>
    <media_control>
    <vc_primitive>
    <to_encoder>
    <picture_fast_update>
    </picture_fast_update>
    </to_encoder>
    </vc_primitive>
    </media_control>

but the code in JsSIP is not set to reply to it -- FreeSwitch keeps sending the INFO message.

I'm not sure what the interaction with WebRTC should be -- a 415 response with `Accept: application/dtmf-relay` looks like the appropriate way to handle this for now.