With the latest from master, this operation:

/socket-binding-group=standard-sockets/socket-binding=http:write-attribute(name=fixed-port,value=true)

results in this operation on the server (incorrect value):

{
"operation" => "write-attribute",
"name" => "fixed-port",
"value" => false,
"address" => [
("socket-binding-group" => "standard-sockets"),
("socket-binding" => "http")
]
}