Fix subprotocol handling

[lpinca]

From https://tools.ietf.org/html/rfc6455#section-4.2.2

/subprotocol/
   Either a single value representing the subprotocol the server
   is ready to use or null.  The value chosen MUST be derived
   from the client's handshake, specifically by selecting one of
   the values from the |Sec-WebSocket-Protocol| field that the
   server is willing to use for this connection (if any).  If the
   client's handshake did not contain such a header field or if
   the server does not agree to any of the client's requested
   subprotocols, the only acceptable value is null.  The absence
   of such a field is equivalent to the null value (meaning that
   if the server does not wish to agree to one of the suggested
   subprotocols, it MUST NOT send back a |Sec-WebSocket-Protocol|
   header field in its response).  The empty string is not the
   same as the null value for these purposes and is not a legal
   value for this field.  The ABNF for the value of this header
   field is (token), where the definitions of constructs and
   rules are as given in [RFC2616].