encodingId syntax

WebRTC 1.0 states: 

```
          RID values passed into <code>init.sendEncodings</code> must be composed only of
          case-sensitive alphanumeric characters (a-z, A-Z, 0-9) up to a maximum of 16
          characters.
```

There is no equivalent syntax restriction for ORTC's encodingId, which is placed into the RID header extension. 
