multiple intermediate headers on redirect

[sckott]

related Ruby webmock issue from 2014 https://github.com/bblimke/webmock/issues/237

We can set a single set of response headers for a stub, but currently not for multiple sets of response headers - crul can now support all intermediate headers, along with httr - so ideally users would be able to stub the set of intermediate headers, not just the final headers returned

interesting that Ruby webmock didn't sort this out