Merge branch 'request-response-header'

* request-response-header method is added.
* Parser callback cannot access to raw header now.
Store redirect headers
More tests on redirects (failing)
Fix request--curl-absolutify-location-history

This fixes redirects test for curl backend
Fix request--url-retrieve-callback
Workaround url.el bug in redirection test
Comment on request-get-simple-redirection
Fix test request-get-simple-redirection
Cosmetic change in request--url-retrieve-callback
Construct history slot in request--url-retrieve-callback
Do not use redirects slot in tests
Remove "redirects" slot from response object
Mention "history" slot instead of "redirects"
Update docstring about parser function
Document request-response-history
Fix request-response-header spec
Document request-response-header