Add flag to exclude instance properties from enumeration

lib/Instance.js

@@ -482,6 +482,14 @@ function Instance(Model, opts) {
 	var addInstanceProperty = function (key) {
 		var defaultValue = null;
 		var prop = Model.allProperties[key];
+		var isEnumerable;
+
+		// Defaults to true unless explicitly set to false
+		isEnumerable = !(
+			prop && prop.hasOwnProperty &&
+			prop.hasOwnProperty('enumerable') &&
+			!prop.enumerable
+		);
 
 		// This code was first added, and then commented out in a later commit.
 		// Its presence doesn't affect tests, so I'm just gonna log if it ever gets called.
@@ -519,7 +527,7 @@ function Instance(Model, opts) {
 					opts.changes.push(key);
 				}
 			},
-			enumerable: true
+			enumerable: isEnumerable
 		});
 	};
 	var addInstanceExtraProperty = function (key) {