refactor(model): simplify some checks

Author: JoaoPedroAS51

Diff for: src/Model.js

@@ -107,7 +107,7 @@ export default class Model extends StaticModel {
     let url = `${this.baseURL()}`;
 
     args.forEach(object => {
-      if (object instanceof Model === false) {
+      if (!(object instanceof Model)) {
         throw new Error('The object referenced on for() method is not a valid Model.')
       }
 
@@ -139,7 +139,7 @@ export default class Model extends StaticModel {
   }
 
   isValidId(id) {
-    return id !== undefined && id !== 0 && id !== '' && id !== null
+    return !!id
   }
 
   endpoint() {