fix: parse.js "parent.add(oneof)“ error (#1602)

Co-authored-by: xiaoweili <[email protected]>

src/namespace.js

@@ -6,6 +6,7 @@ var ReflectionObject = require("./object");
 ((Namespace.prototype = Object.create(ReflectionObject.prototype)).constructor = Namespace).className = "Namespace";
 
 var Field    = require("./field"),
+    OneOf    = require("./oneof"),
     util     = require("./util");
 
 var Type,    // cyclic
@@ -217,7 +218,7 @@ Namespace.prototype.getEnum = function getEnum(name) {
  */
 Namespace.prototype.add = function add(object) {
 
-    if (!(object instanceof Field && object.extend !== undefined || object instanceof Type || object instanceof Enum || object instanceof Service || object instanceof Namespace))
+    if (!(object instanceof Field && object.extend !== undefined || object instanceof Type || object instanceof Enum || object instanceof Service || object instanceof Namespace || object instanceof OneOf))
         throw TypeError("object must be a valid nested object");
 
     if (!this.nested)