Suggestion: `seeResponseIsPdf()`

[ThomasLandauer]

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:

$I->seeHttpHeader('Content-Type', 'application/pdf');
$I->seeResponseContains('%PDF-1.');

REST module has seeResponseIsJson() and seeResponseIsXml()