fix(model): use object as options.scope default

lib/model.js

@@ -869,7 +869,7 @@ class Model {
       schema: null,
       schemaDelimiter: '',
       defaultScope: {},
-      scopes: [],
+      scopes: {},
       indexes: []
     }, options);
 

test/unit/associations/belongs-to-many.test.js

@@ -45,7 +45,7 @@ describe(Support.getTestDialectTeaser('belongsToMany'), () => {
     const AB = current.model('AB');
 
     expect(AB.options.defaultScope).to.deep.equal({});
-    expect(AB.options.scopes).to.have.length(0);
+    expect(AB.options.scopes).to.deep.equal({});
   });
 
   it('should not inherit validations from parent to join table', () => {