approve failing test

test/token/ERC20/SafeERC20.test.js

@@ -99,6 +99,10 @@ contract('SafeERC20', function () {
     });
   });
 
+  it('should throw while approving with non-zero existing allowance', async function () {
+    await shouldFail.reverting(this.helper.doFailingApproveByValue());
+  });
+
   it('should not throw on succeeding increaseAllowance', async function () {
     await this.helper.doSucceedingIncreaseAllowance();
   });