Suggestion: `seeResponseIsPdf()`

It would be nice to be able to assert that a response is indeed a PDF.
Right now, I'm doing it with something like:
```php
$I->seeHttpHeader('Content-Type', 'application/pdf');
$I->seeResponseContains('%PDF-1.');
```
REST module has [`seeResponseIsJson()`](https://codeception.com/docs/modules/REST#seeResponseIsJson) and [`seeResponseIsXml()`](https://codeception.com/docs/modules/REST#seeResponseIsXml)