Messages - 3.3.1 Change Schema parameter to required.

Change to :

REQUIRED. The schema in which the data is to be returned. The default value is //openid//. If the value of this parameter is omitted, or not //openid//, the response may be a proprietary schema to support backwards compatibility. A URL MAY be passed to define custom schemes not specified by short names. Custom schema names and responses are out of scope for this specification.