Close on unsupported client protocol.

Author: RubenVerborgh

api-websockets.md

@@ -65,6 +65,16 @@ the server SHOULD warn the client as follows:
 warning Missing Sec-WebSocket-Protocol header, expected value 'solid-ws-draft/v0.1.0-alpha'
 ```
 
+Otherwise, if the set of values obtained
+from parsing the `Sec-WebSocket-Protocol` header
+does not contain `solid-ws-draft/v0.1.0-alpha`,
+then the server SHOULD emit a warning
+and SHOULD close the connection:
+
+```
+error Client does not support protocol solid-ws-draft/v0.1.0-alpha
+```
+
 ### Subscription
 Then, the client needs to *sub*(scribe) to a given resource URI.
 If any change occurs in that resource,