HTTP PUT com.mycompany.myapp.Order|123/orderStatus
Request body
{
"value": ...
}
Request headers:
- If-Match: (value of timestamp)
- where timestamp is as obtained from object representation
Request validation only 
custom header "X-ValidateOnly": "true"
Response if the value is valid
modifies the value
returns: 200 ok
Response if the value is invalid
- some sort of 4xx error
- Response 'Warning' header: error message
- ??? additionally, updated representation that has a message against the invalid value ???
Response if object cannot be found
returns: 404
Response if the object has changed
returns: 409 "conflict"
header: "Last-Modified"
Update existing object with new values.