Assert::integerish() with floats/numeric will always evaluate to false

Calling `Assert::integerish(1.000)` with a float or numeric the following error is reported:

`Call to static method Webmozart\Assert\Assert::integerish() with float will always evaluate to false.`

```php
Assert::integerish(1.000); // OK
Assert::integerish(1.001); // not OK
```
