ID must be present in both request and response

from the spec:
This member is REQUIRED.
It MUST be the same as the value of the id member in the Request Object.
If there was an error in detecting the id in the Request object (e.g. Parse error/Invalid Request), it MUST be Null.

rpc/lib/types/types.go

@@ -62,7 +62,7 @@ type RPCError struct {
 
 type RPCResponse struct {
 	JSONRPC string           `json:"jsonrpc"`
-	ID      string           `json:"id,omitempty"`
+	ID      string           `json:"id"`
 	Result  *json.RawMessage `json:"result,omitempty"`
 	Error   *RPCError        `json:"error,omitempty"`
 }