Remove JSON field tags from the jmessage type.

Fix a case where the client wasn't marshaling correctly in the response to a
callback.

json.go

@@ -94,16 +94,16 @@ func (j *jmessages) parseJSON(data []byte) error {
 
 // jmessage is the transmission format of a protocol message.
 type jmessage struct {
-	V  string          `json:"jsonrpc"`      // must be Version
-	ID json.RawMessage `json:"id,omitempty"` // may be nil
+	V  string          // must be Version
+	ID json.RawMessage // may be nil
 
 	// Fields belonging to request or notification objects
-	M string          `json:"method,omitempty"`
-	P json.RawMessage `json:"params,omitempty"` // may be nil
+	M string
+	P json.RawMessage // may be nil
 
 	// Fields belonging to response or error objects
-	E *Error          `json:"error,omitempty"`  // set on error
-	R json.RawMessage `json:"result,omitempty"` // set on success
+	E *Error          // set on error
+	R json.RawMessage // set on success
 
 	// N.B.: In a valid protocol message, M and P are mutually exclusive with E
 	// and R. Specifically, if M != "" then E and R must both be unset. This is

opts.go

@@ -253,7 +253,7 @@ func (c *ClientOptions) handleCallback() func(*jmessage) []byte {
 				rsp.E = &Error{Code: code.FromError(err), Message: err.Error()}
 			}
 		}
-		bits, _ := json.Marshal(rsp)
+		bits, _ := rsp.toJSON()
 		return bits
 	}
 }