Added mongoose.Types.ObjectId.toHexString()

mongoose/mongoose-tests.ts

@@ -364,3 +364,5 @@ schema.virtual('display_name')
   .get(function(): string { return this.name; })
   .set((value: string): void => {});
 
+var id : mongoose.Types.ObjectId;
+var s = id.toHexString();

mongoose/mongoose.d.ts

@@ -78,7 +78,9 @@ declare module "mongoose" {
     set(fn: Function): VirtualType;
   }
   export module Types {
-    export class ObjectId {}
+    export class ObjectId {
+        toHexString(): string;
+    }
   }
 
   export class Schema {