Only copy original request headers when following redirects [fixes #4790]

When following a redirect, the client should not add NULL headers
from the follow_headers list that were not set on the original
request. This prevents problems when requesting URLs that
include an authorization signature calculated from the request URL
and headers.
Merge pull request #401 from acoulton/3.3/bug/4790-extra-headers-on-redirect-follow

Only copy original request headers when following redirects [fixes #4790]