HTTP GET com.mycompany.myapp.Order|123/placeOrder/invoke
Request body
{
"param1": ...,
"param2": ...,
"param3": ...
}
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 arguments are valid
- invokes the action
- returns: 200
- response: for an object or a collection
Response if the arguments are 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-At
Response if the action is not idempotent
returns: 405 "Method Not Allowed"