fix(optimistic_locking): increment version attribute with Model.decrement (#8593)

lib/model.js

@@ -2926,7 +2926,7 @@ class Model {
       options.attributes[updatedAtAttribute.field || updatedAtAttr] = this._getDefaultTimestamp(updatedAtAttr) || Utils.now(this.sequelize.options.dialect);
     }
     if (versionAttr) {
-      values[versionAttr] = 1;
+      values[versionAttr] = options.increment ? 1 : -1;
     }
 
     for (const attr of Object.keys(values)) {

test/integration/model/optimistic_locking.test.js

@@ -65,5 +65,16 @@ describe(Support.getTestDialectTeaser('Model'), () => {
         expect(account.version).to.eq(1);
       });
     });
+
+    it('decrement() also increments the version', () => {
+      return Account.create({number: 1}).then(account => {
+        expect(account.version).to.eq(0);
+        return account.decrement('number', { by: 1} );
+      }).then(account => {
+        return account.reload();
+      }).then(account => {
+        expect(account.version).to.eq(1);
+      });
+    });
   });
 });