HTTP PUT com.mycompany.myapp.Order|123
Update existing object with new values.
if all values are valid
persists the object
return 200: OK
if any value is invalid
- some sort of 4xx error
- Response 'Warning' header: error message
- ??? additionally, updated representation that has a message against the invalid value ???
if object cannot be found
returns: 404
if object has been updated
returns: 409 "conflict"
validation only 
custom header "X-ValidateOnly": "true"