created a method isPost in Request

so that we don't repeat the same long test everywhere in the code


Note that there is two places where it's still not changed here's why 

./admin/controller/marketing/coupon.php line 388  and ./admin/controller/sale/voucher.php also line 388 

```
if (isset($this->request->get['coupon_id']) && (!$this->request->server['REQUEST_METHOD'] != 'POST'))
```

I'm confused by the double negation ...  I've checked in the history and it has been here since the import on github....