TURN server should return 420 (Unknown attribute) when it sees unexpected attribute

[enobufs]

I noticed that when a request contained Requested-Transport attribute in CreatePermission, RefreshRequest or ChannelBind request, the TURN server never complains it and handles it as if it was no there.

The coturn however, returns 420 (Unknown Attribute) in the above cases.

Our TURN client is being tested with its own TURN server, it would be great to generate errors when the server sees unexpected situations.