Expose previous response headers to beforeRedirect callback

[aivils]

Of course, the set-cookie should not be considered for this implementation. This implementation must allow the use of set-cookie header.
Current beforeRedirect implementation is too restrictive.
https://github.com/follow-redirects/follow-redirects/blob/master/index.js#L356
It does not allow to access headers of response in the beforeRedirect function.

[RubenVerborgh]

Ah wait, I got confused by the title and the last comment, but the original issue makes a valid point. Let me restore.