Properly convert results for Puppeteer version 23 (solves discussion #866)

[JeppeKnockaert]

Since Puppeteer 23.0.0, the result returned by Puppeteer is a Uint8Array instead of a Buffer (see puppeteer/puppeteer#12823 and https://github.com/puppeteer/puppeteer/releases/tag/puppeteer-core-v23.0.0).

They state that most code should still work as Node APIs often accept Uint8Arrays. The toString method used by Browsershot requires a Buffer, so we need to convert the result to a Buffer first.

The test I've added succeeds with Puppeteer v22 and v23. In v23 it fails without the change because base64_decode returns false on an invalid base64 string, in v22 it succeeds with or without the change.

The failing tests were introduced by an earlier PR and should be fixed in #871 or #873

This solves this discussion: #866

[freekmurze]

Fixed by #870

[jan-tricks]

@freekmurze can you take another look at this? This PR checks whether the result is an array (and thus comes from Puppeteer 23) or whether it is already a buffer. Since the check was lacking from the merged PR we got the white pages on Puppeteer < 23.